the interior of a motor vehicle engine shows that ordinarily the
lubricant is not as much to blame as is commonly supposed. The analysis
was as follows:
Oil 14.3%
Other combustible matter 17.9
Sand, clay, etc. 24.8
Iron oxide 24.5
Carbonate of lime 8.9
Other constituents 9.6
It is extremely probable that the above could be divided into two
general classes, these being approximately 32.2% oil and combustible
matter and a much larger proportion, or 67.8% of earthy matter. The
presence of such a large percentage of earthy matter is undoubtedly due
to the impurities in the air, such as road dust which has been sucked
in through the carburetor. The fact that over 17% of the matter which is
combustible was not of an oily nature lends strong support to this view.
There would not be the amount of earthy material present in the carbon
deposits of an airplane engine as above stated because the air is almost
free from dust at the high altitudes planes are usually flown. One could
expect to find more combustible and less earthy matter and the carbon
would be softer and more easily removed. It is very good practice to
provide a screen on the air intake to reduce the amounts of dust sucked
in with the air as well as observing the proper precautions relative to
supplying the proper quantities of air to the mixture and of not using
any more oil than is needed to insure proper lubrication of the internal
mechanism.
USE OF CARBON SCRAPERS
It is not unusual for one to hear an aviator complain that the engine he
operates is not as responsive as it was when new after he has run it but
relatively few hours. There does not seem to be anything actually wrong
with the engine, yet it does not respond readily to the throttle and is
apt to overheat. While these symptoms denote a rundown condition of the
mechanism, the trouble is often due to nothing more serious than
accumulations of carbon. The remedy is the removal of this matter out of
place. The surest way of cleaning the inside of the motor thoroughly is
to remove the cylinders, if these members are cast integrally with the
head or of removing the head member if that is a separate casting, to
expose all parts.
In certain forms of cylinders, especially those of the L form, it is
possible to introduce simple scrapers down through the valve chamber cap
holes and through the spark-plug hole if this component is placed in the
cylinder in some position that communicates directly to the interior of
the cylinder or to the piston top. No claim can be made for originality
or novelty of this process as is has been used for many years on large
stationary engines. The first step is to dismantle the inlet and exhaust
piping and remove the valve caps and valves, although if the deposit is
not extremely hard or present in large quantities one can often
manipulate the scrapers in the valve cap openings without removing
either the piping or the valves. Commencing with the first cylinder, the
crank-shaft is turned till the piston is at the top of its stroke, then
the scraper may be inserted, and the operation of removing the carbon
started by drawing the tool toward the opening. As this is similar to a
small hoe, the cutting edge will loosen some of the carbon and will draw
it toward the opening. A swab is made of a piece of cloth or waste
fastened at the end of a wire and well soaked in kerosene to clean out
the cylinder.
When available, an electric motor with a length of flexible shaft and a
small circular cleaning brush having wire bristles can be used in the
interior of the engine. The electric motor need not be over one-eighth
horse-power running 1,200 to 1,600 R. P. M., and the wire brush must, of
course, be of such size that it can be easily inserted through the valve
chamber cap. The flexible shaft permits one to reach nearly all parts of
the cylinder interior without difficulty and the spreading out and
flattening of the brush insures that considerable surface will be
covered by that member.
BURNING OUT CARBON WITH OXYGEN
A process of recent development that gives very good results in removing
carbon without disassembling the motor depends on the process of burning
out that material by supplying oxygen to support the combustion and to
make it energetic. A number of concerns are already offering apparatus
to accomplish this work, and in fact any shop using an autogenous
welding outfit may use the oxygen tank and reducing valve in connection
with a simple special torch for burning the carbon. Results have
demonstrated that there is little danger of damaging the motor parts,
and that the cost of oxygen and labor is much lower than the old method
of removing the cylinders and scraping the carbon out, as well as being
very much quicker than the alternative process of using carbon solvent.
The only drawback to this system is that there is no absolute insurance
that every particle of carbon will be removed, as small protruding
particles may be left at points that the flame does not reach and cause
pre-ignition and consequent pounding, even after the oxygen treatment.
It is generally known that carbon will burn in the presence of oxygen,
which supports combustion of all materials, and this process takes
advantage of this fact and causes the gas to be injected into the
combustion chamber over a flame obtained by a match or wax taper.
[Illustration: Fig. 182.--Showing Where Carbon Deposits Collect in
Engine Combustion Chamber, and How to Burn Them Out with the Aid of
Oxygen. A--Special Torch. B--Torch Coupled to Oxygen Tank. C--Torch in
Use.]
It is suggested by those favoring this process that the night before the
oxygen is to be used the engine be given a conventional kerosene
treatment. A half tumbler full of this liquid or of denatured alcohol is
to be poured into each cylinder and permitted to remain there over
night. As a precaution against fire, the gasoline is shut off from the
carburetor before the torch is inserted in the cylinder and the motor
started so that the gasoline in the pipe and carburetor float chamber
will be consumed. Work is done on one cylinder at a time. A note of
caution was recently sounded by a prominent spark-plug manufacturer
recommending that the igniter member be removed from the cylinder in
order not to injure it by the heat developed. The outfits on the market
consist of a special torch having a trigger controlled valve and a
length of flexible tubing such as shown at Fig. 182, A, and a regulating
valve and oxygen tank as shown at B. The gauge should be made to
register about twelve pounds pressure.
The method of operation is very simple and is outlined at C. The burner
tube is placed in the cylinder and the trigger valve is opened and the
oxygen permitted to circulate in the combustion chamber. A lighted match
or wax taper is dropped in the chamber and the injector tube is moved
around as much as possible so as to cover a large area. The carbon takes
fire and burns briskly in the presence of the oxygen. The combustion of
the carbon is accompanied by sparks and sometimes by flame if the
deposit is of an oily nature. Once the carbon begins to burn the
combustion continues without interruption as long as the oxygen flows
into the cylinder. Full instructions accompany each outfit and the
amount of pressure for which the regulator should be set depends upon
the design of the torch and the amount of oxygen contained in the
storage tank.
REPAIRING SCORED CYLINDERS
If the engine has been run at any time without adequate lubrication, one
or more of the cylinders may be found to have vertical scratches running
up and down the cylinder walls. The depth of these will vary according
to the amount of time the cylinder was without lubrication, and if the
grooves are very deep the only remedy is to purchase a new member. Of
course, if sufficient stock is available in the cylinder walls, the
cylinders may be rebored and new pistons which are oversize, _i.e._,
larger than standard, may be fitted. Where the scratches are not deep
they may be ground out with a high speed emery wheel or lapped out if
that type of machine is not available. Wrist pins have been known to
come loose, especially when these are retained by set screws that are
not properly locked, and as wrist-pins are usually of hardened steel it
will be evident that the sharp edge of that member can act as a cutting
tool and make a pronounced groove in the cylinder. Cylinder grinding is
a job that requires skilled mechanics, but may be accomplished on any
lathe fitted with an internal grinding attachment. While automobile
engine cylinders usually have sufficient wall thickness to stand
reboring, those of airplane engines seldom have sufficient metal to
permit of enlarging the bore very much by a boring tool. A few
thousandths of an inch may be ground out without danger, however. An
airplane engine cylinder with deep grooves must be scrapped as a general
rule.
Where the grooves in the cylinder are not deep or where it has warped
enough so the rings do not bear equally at all parts of the cylinder
bore, it is possible to obtain a fairly accurate degree of finish by a
lapping process in which an old piston is coated with a mixture of fine
emery and oil and is reciprocated up and down in the cylinder as well as
turned at the same time. This may be easily done by using a dummy
connecting rod having only a wrist pin end boss, and of such size at the
other end so that it can be held in the chuck of a drill press. The
cylinder casting is firmly clamped on the drill press table by suitable
clamping blocks, and a wooden block is placed in the combustion chamber
to provide a stop for the piston at its lower extreme position. The back
gears are put in and the drill chuck is revolved slowly. All the while
that the piston is turning the drill chuck should be raised up and down
by the hand feed lever, as the best results are obtained when the
lapping member is given a combination of rotary and reciprocating
motion.
VALVE REMOVAL AND INSPECTION
One of the most important parts of the gasoline engine and one that
requires frequent inspection and refitting to keep in condition, is the
mushroom or poppet valve that controls the inlet and exhaust gas flow.
In overhauling it is essential that these valves be removed from their
seatings and examined carefully for various defects which will be
enumerated at proper time. The problem that concerns us now is the best
method of removing the valve. These are held against the seating in the
cylinder by a coil spring which exerts its pressure on the cylinder
casting at the upper end and against a suitable collar held by a key at
the lower end of the valve stem. In order to remove the valve it is
necessary to first compress the spring by raising the collar and pulling
the retaining key out of the valve stem. Many forms of valve spring
lifters have been designed to permit ready removal of the valves.
When the cylinder is of the valve in-the-head form, the method of valve
removal will depend entirely upon the system of cylinder construction
followed. In the Sturtevant cylinder design it is possible to remove the
head from the cylinder castings and the valve springs may be easily
compressed by any suitable means when the cylinder head is placed on the
work bench where it can be easily worked on. The usual method is to
place the head on a soft cloth with the valves bearing against the
bench. The valve springs may then be easily pushed down with a simple
forked lever and the valve stem key removed to release the valve spring
collar. In the Curtiss OX-2 (see Fig. 182-1/2) and Hall-Scott engines it
is not possible to remove the valves without taking the cylinder off
the crank-case, because the valve seats are machined directly in the
cylinder head and the valve domes are cast integrally with the cylinder.
This means that if the valves need grinding the cylinder must be removed
from the engine base to provide access to the valve heads which are
inside of that member, and which cannot be reached from the outside as
is true of the L-cylinder construction. In the Curtiss VX engines, the
valves are carried in detachable cages which may be removed when the
valves need attention.
[Illustration: Fig. 182-1/2.--Part Sectional View, Showing Valve
Arrangement in Cylinder of Curtiss OX-2 Aviation Engine.]
RESEATING AND TRUING VALVES
Much has been said relative to valve grinding, and despite the mass of
information given in the trade prints it is rather amusing to watch the
average repairman or the engine user who prides himself on maintaining
his own motor performing this essential operation. The common mistakes
are attempting to seat a badly grooved or pitted valve head on an
equally bad seat, which is an almost hopeless job, and of using coarse
emery and bearing down with all one's weight on the grinding tool with
the hope of quickly wearing away the rough surfaces. The use of improper
abrasive material is a fertile cause of failure to obtain a satisfactory
seating. Valve grinding is not a difficult operation if certain
precautions are taken before undertaking the work. The most important of
these is to ascertain if the valve head or seat is badly scored or
pitted. If such is found to be the case no ordinary amount of grinding
will serve to restore the surfaces. In this event the best thing to do
is to remove the valve from its seating and to smooth down both the
valve head and the seat in the cylinder before attempt is made to fit
them together by grinding. Another important precaution is to make sure
that the valve stem is straight, and that the head is not warped out of
shape.
[Illustration: Fig. 183.--Tools for Restoring Valve Head and Seats.]
A number of simple tools is available at the present time for reseating
valves, these being outlined at Fig. 183. That shown at A is a simple
fixture for facing off the valve head. The stem is supported by suitable
bearings carried by the body or shank of the tool, and the head is
turned against an angularly disposed cutter which is set for the proper
valve seat angle. The valve head is turned by a screw-driver, the amount
of stock removed from the head depending upon the location of the
adjusting screw. Care must be taken not to remove too much metal, only
enough being taken off to remove the most of the roughness. Valves are
made in two standard tapers, the angle being either 45 or 60 degrees. It
is imperative that the cutter blade be set correctly in order that the
bevel is not changed. A set of valve truing and valve-seat reaming
cutters is shown at Fig. 183, B. This is adaptable to various size valve
heads, as the cutter blade D may be moved to correspond to the size of
the valve head being trued up. These cutter blades are made of tool
steel and have a bevel at each end, one at 45 degrees, the other at 60
degrees. The valve seat reamer shown at G will take any one of the heads
shown at F. It will also take any one of the guide bars shown at H. The
function of the guide bars is to fit the valve stem bearing in order to
locate the reamer accurately and to insure that the valve seat is
machined concentrically with its normal center. Another form of valve
seat reamer and a special wrench used to turn it is shown at C. The
valve head truer shown at Fig. 183, D, is intended to be placed in a
vise and is adaptable to a variety of valve head sizes. The smaller
valves merely fit deeper in the conical depression. The cutter blade is
adjustable and the valve stem is supported by a simple self-centering
bearing. In operation it is intended that the valve stem, which
protrudes through the lower portion of the guide bearing, shall be
turned by a drill press or bit stock while the valve head is set against
the cutter by pressure of a pad carried at the end of a feed screw which
is supported by a hinged bridge member. This can be swung out of place
as indicated to permit placing the valve head against the cutter or
removing it.
As the sizes of valve heads and stems vary considerably a "Universal"
valve head truing tool must have some simple means of centering the
valve stem in order to insure concentric machining of the valve head. A
valve head truer which employs an ingenious method of guiding the valve
stem is shown at Fig. 183, E. The device consists of a body portion, B,
provided with an external thread at the top on which the cutter head, A,
is screwed. A number of steel balls, C, are carried in the grooves which
may be altered in size by the adjustment nut, F, which screws in the
bottom of the body portion, B. As the nut F is screwed in against the
spacer member E, the V-grooves are reduced in size and the steel balls,
C, are pressed out in contact with the valve stem. As the circle or
annulus is filled with balls in both upper and lower portions the stem
may be readily turned because it is virtually supported by ball bearing
guides. When a larger valve stem is to be supported, the adjusting nut
F, is screwed out which increases the size of the grooves and permits
the balls, C, to spread out and allow the larger stem to be inserted.
VALVE GRINDING PROCESSES
Mention has been previously made of the importance of truing both valve
head and seat before attempt is made to refit the parts by grinding.
After smoothing the valve seat the next step is to find some way of
turning the valve. Valve heads are usually provided with a screw-driver
slot passing through the boss at the top of the valve or with two
drilled holes to take a forked grinding tool. A combination grinding
tool has been devised which may be used when either the two drilled
holes or the slotted head form of valve is to be rotated. This consists
of a special form of screw driver having an enlarged boss just above the
blade, this boss serving to support a U-shape piece which can be
securely held in operative position by the clamp screw or which can be
turned out of the way if the screw driver blade is to be used.
As it is desirable to turn the valve through a portion of a revolution
and back again rather than turning it always in the same direction, a
number of special tools has been designed to make this oscillating
motion possible without trouble. A simple valve grinding tool is shown
at Fig. 184, C. This consists of a screw-driver blade mounted in a
handle in such a way that the end may turn freely in the handle. A
pinion is securely fastened to the screw-driver blade shank, and is
adapted to fit a race provided with a wood handle and guided by a bent
bearing member securely fastened to the screw-driver handle. As the rack
is pushed back and forth the pinion must be turned first in one
direction and then in the other.
[Illustration: Fig. 184.--Tools and Processes Utilized in Valve
Grinding.]
A valve grinding tool patterned largely after a breast drill is shown at
Fig. 184, D. This is worked in such a manner that a continuous rotation
of the operating crank will result in an oscillating movement of the
chuck carrying the screw-driver blade. The bevel pinions which are used
to turn the chuck are normally free unless clutched to the chuck stem by
the sliding sleeve which must turn with the chuck stem and which carries
clutching members at each end to engage similar members on the bevel
pinions and lock these to the chuck stem, one at a time. The bevel gear
carries a cam-piece which moves the clutch sleeve back and forth as it
revolves. This means that the pinion giving forward motion of the chuck
is clutched to the chuck spindle for a portion of a revolution of the
gear and clutch sleeve is moved back by the cam and clutched to the
pinion giving a reverse motion of the chuck during the remainder of the
main drive gear revolution.
It sometimes happens that the adjusting screw on the valve lift plunger
or the valve lift plunger itself when L head cylinders are used does not
permit the valve head to rest against the seat. It will be apparent that
unless a definite space exists between the end of the valve stem and the
valve lift plunger that grinding will be of little avail because the
valve head will not bear properly against the abrasive material smeared
on the valve seat.
The usual methods of valve grinding are clearly outlined at Fig. 184.
The view at the left shows the method of turning the valve by an
ordinary screw driver and also shows a valve head at A, having both the
drilled holes and the screw-driver slot for turning the member and two
special forms of fork-end valve grinding tools. In the sectional view
shown at the right, the use of the light spring between the valve head
and the bottom of the valve chamber to lift the valve head from the seat
whenever pressure on the grinding tool is released is clearly indicated.
It will be noted also that a ball of waste or cloth is interposed in the
passage between the valve chamber and the cylinder interior to prevent
the abrasive material from passing into the cylinder from the valve
chamber. When a bitstock is used, instead of being given a true rotary
motion the chuck is merely oscillated through the greater part of the
circle and back again. It is necessary to lift the valve from its seat
frequently as the grinding operation continues; this is to provide an
even distribution of the abrasive material placed between the valve head
and its seat. Only sufficient pressure is given to the bitstock to
overcome the uplift of the spring and to insure that the valve will be
held against the seat. Where the spring is not used it is possible to
raise the valve from time to time with the hand which is placed under
the valve stem to raise it as the grinding is carried on. It is not
always possible to lift the valve in this manner when the cylinders are
in place on the engine base owing to the space between the valve lift
plunger and the end of the valve stem. In this event the use of the
spring as shown in sectional view will be desirable.
The abrasive generally used is a paste made of medium or fine emery and
lard oil or kerosene. This is used until the surfaces are comparatively
smooth, after which the final polish or finish is given with a paste of
flour emery, grindstone dust, crocus, or ground glass and oil. An
erroneous impression prevails in some quarters that the valve head
surface and the seating must have a mirror-like polish. While this is
not necessary it is essential that the seat in the cylinder and the
bevel surface of the head be smooth and free from pits or scratches at
the completion of the operation. All traces of the emery and oil should
be thoroughly washed out of the valve chamber with gasoline before the
valve mechanism is assembled and in fact it is advisable to remove the
old grinding compound at regular intervals, wash the seat thoroughly and
supply fresh material as the process is in progress.
The truth of seatings may be tested by taking some Prussian blue pigment
and spreading a thin film of it over the valve seat. The valve is
dropped in place and is given about one-eighth turn with a little
pressure on the tool. If the seating is good both valve head and seat
will be covered uniformly with color. If high spots exist, the heavy
deposit of color will show these while the low spots will be made
evident because of the lack of pigment. The grinding process should be
continued until the test shows an even bearing of the valve head at all
points of the cylinder seating. When the valves are held in cages it is
possible to catch the cage in a vise and to turn the valve in any of the
ways indicated. It is much easier to clean off the emery and oil and
there is absolutely no danger of getting the abrasive material in the
cylinder if the construction is such that the valve cage or cylinder
head member carrying the valve can be removed from the cylinder. When
valves are held in cages, the tightness of the seat may be tested by
partially filling the cage with gasoline and noticing how much liquid
oozes out around the valve head. The degree of moisture present
indicates the efficacy of the grinding process.
The valves of Curtiss OX-2 cylinders are easily ground in by using a
simple fixture or tool and working from the top of the cylinder instead
of from the inside. A tube having a bore just large enough to go over
the valve stem is provided with a wooden handle or taped at one end and
a hole of the same size as that drilled through the valve stem is put in
at the other. To use, the open end of the tube is pushed over the valve
stem and a split pin pushed through the tube and stem. The valve may be
easily manipulated and ground in place by oscillating in the customary
manner.
DEPRECIATION IN VALVE OPERATING SYSTEMS
There are a number of points to be watched in the valve operating system
because valve timing may be seriously interfered with if there is much
lost motion at the various bearing points in the valve lift mechanism.
The two conventional methods of opening valves are shown at Fig. 185.
That at A is the type employed when the valve cages are mounted directly
in the head, while the form at B is the system used when the valves are
located in a pocket or extension of the cylinder casting as is the case
if an L, or T-head cylinder is used. It will be evident that there are
several points where depreciation may take place. The simplest form is
that shown at B, and even on this there are five points where lost
motion may be noted. The periphery of the valve opening cam or roller
may be worn, though this is not likely unless the roller or cam has
been inadvertently left soft. The pin which acts as a bearing for the
roller may become worn, this occurring quite often. Looseness may
materialize between the bearing surfaces of the valve lift plunger and
the plunger guide casting, and there may also be excessive clearance
between the top of the plunger and the valve stem.
[Illustration: Fig. 185.--Outlining Points in Valve Operating Mechanism
Where Depreciation is Apt to Exist.]
On the form shown at A, there are several parts added to those indicated
at B. A walking beam or rocker lever is necessary to transform the
upward motion of the tappet rod to a downward motion of the valve stem.
The pin on which this member fulcrums may wear as will also the other
pin acting as a hinge or bearing for the yoke end of the tappet rod. It
will be apparent that if slight play existed at each of the points
mentioned it might result in a serious diminution of valve opening.
Suppose, for example, that there were .005-inch lost motion at each of
three bearing points, the total lost motion would be .015-inch or
sufficient to produce noisy action of the valve mechanism. When valve
plungers of the adjustable form, such as shown at B, are used, the
hardened bolt head in contact with the end of the valve stem may become
hollowed out on account of the hammering action at that point. It is
imperative that the top of this member be ground off true and the
clearance between the valve stem and plunger properly adjusted. If the
plunger is a non-adjustable type it will be necessary to lengthen the
valve stem by some means in order to reduce the excessive clearance. The
only remedy for wear at the various hinges and bearing pins is to bore
the holes out slightly larger and to fit new hardened steel pins of
larger diameter. Depreciation between the valve plunger guide and the
valve plunger is usually remedied by fitting new plunger guides in place
of the worn ones. If there is sufficient stock in the plunger guide
casting as is sometimes the case when these members are not separable
from the cylinder casting, the guide may be bored out and bushed with a
light bronze bushing.
A common cause of irregular engine operation is due to a sticking valve.
This may be owing to a bent valve stem, a weak or broken valve spring or
an accumulation of burnt or gummed oil between the valve stem and the
valve stem guide. In order to prevent this the valve stem must be
smoothed with fine emery cloth and no burrs or shoulders allowed to
remain on it, and the stem must also be straight and at right angles to
the valve head. If the spring is weak it may be strengthened in some
cases by stretching it out after annealing so that a larger space will
exist between the coils and re-hardening. Obviously if a spring is
broken the only remedy is replacement of the defective member.
Mention has been made of wear in the valve stem guide and its influence
on engine action. When these members are an integral part of the
cylinder the only method of compensating for this wear is to drill the
guide out and fit a bushing, which may be made of steel tube.
In some engines, especially those of recent development, the valve stem
guide is driven or screwed into the cylinder casting and is a separate
member which may be removed when worn and replaced with a new one. When
the guides become enlarged to such a point that considerable play exists
between them and the valve stems, they may be easily knocked out or
unscrewed.
PISTON TROUBLES
If an engine has been entirely dismantled it is very easy to examine the
pistons for deterioration. While it is important that the piston be a
good fit in the cylinder it is mainly upon the piston rings that
compression depends. The piston should fit the cylinder with but little
looseness, the usual practice being to have the piston about .001-inch
smaller than the bore for each inch of piston diameter at the point
where the least heat is present or at the bottom of the piston. It is
necessary to allow more than this at the top of the piston owing to its
expansion due to the direct heat of the explosion. The clearance is
usually graduated and a piston that would be .005-inch smaller than the
cylinder bore at the bottom would be about .0065-inch at the middle and
.0075-inch at the top. If much more play than this is evidenced the
piston will "slap" in the cylinder and the piston will be worn at the
ends more than in the center. Aluminum or alloy pistons require more
clearance than cast iron ones do, usually 1.50 times as much. Pistons
sometimes warp out of shape and are not truly cylindrical. This results
in the high spots rubbing on the cylinder while the low spots will be
blackened where a certain amount of gas has leaked by.
Mention has been previously made of the necessity of reboring or
regrinding a cylinder that has become scored or scratched and which
allows the gas to leak by the piston rings. When the cylinder is ground
out, it is necessary to use a larger piston to conform to the enlarged
cylinder bore. Most manufacturers are prepared to furnish over-size
pistons, there being four standard over-size dimensions adopted by the
S. A. E. for rebored cylinders. These are .010-inch, .020-inch,
.030-inch, and .040-inch larger than the original bore.
The piston rings should be taken out of the piston grooves and all
carbon deposits removed from the inside of the ring and the bottom of
the groove. It is important to take this deposit out because it prevents
the rings from performing their proper functions by reducing the ring
elasticity, and if the deposit is allowed to accumulate it may
eventually result in sticking and binding of the ring, this producing
excessive friction or loss of compression. When the rings are removed
they should be tested to see if they retain their elasticity and it is
also well to see that the small pins in some pistons which keep the
rings from turning around so the joints will not come in line are still
in place. If no pins are found there is no cause for alarm because these
dowels are not always used. When fitted, they are utilized with rings
having a butt joint or diagonal cut as the superior gas retaining
qualities of the lap or step joint render the pins unnecessary.
If gas has been blowing by the ring or if these members have not been
fitting the cylinder properly the points where the gas passed will be
evidenced by burnt, brown or roughened portions of the polished surface
of the pistons and rings. The point where this discoloration will be
noticed more often is at the thin end of an eccentric ring, the
discoloration being present for about 1/2-inch or 3/4-inch each side of
the slot. It may be possible that the rings were not true when first
put in. This made it possible for the gas to leak by in small amounts
initially which increased due to continued pressure until quite a large
area for gas escape had been created.
PISTON RING MANIPULATION
Removing piston rings without breaking them is a difficult operation if
the proper means are not taken, but is a comparatively simple one when
the trick is known. The tools required are very simple, being three
strips of thin steel about one-quarter inch wide and four or five inches
long and a pair of spreading tongs made up of one-quarter inch diameter
keystock tied in the center with a copper wire to form a hinge. The
construction is such that when the hand is closed and the handles
brought together the other end of the expander spreads out, an action
just opposite to that of the conventional pliers. The method of using
the tongs and the metal strips is clearly indicated at Fig. 186. At A
the ring expander is shown spreading the ends of the rings sufficiently
to insert the pieces of sheet metal between one of the rings and the
piston. Grasp the ring as shown at B, pressing with the thumbs on the
top of the piston and the ring will slide off easily, the thin metal
strips acting as guide members to prevent the ring from catching in the
other piston grooves. Usually no difficulty is experienced in removing
the top or bottom rings, as these members may be easily expanded and
worked off directly without the use of a metal strip. When removing the
intermediate rings, however, the metal strips will be found very useful.
These are usually made by the repairman by grinding the teeth from old
hacksaw blades and rounding the edges and corners in order to reduce the
liability of cutting the fingers. By the use of the three metal strips a
ring is removed without breaking or distorting it and practically no
time is consumed in the operation.
FITTING PISTON RINGS
Before installing new rings, they should be carefully fitted to the
grooves to which they are applied. The tools required are a large piece
of fine emery cloth, a thin, flat file, a small vise with copper or
leaden jaw clips, and a smooth hard surface such as that afforded by the
top of a surface plate or a well planed piece of hard wood. After making
sure that all deposits of burnt oil and carbon have been removed from
the piston grooves, three rings are selected, one for each groove. The
ring is turned all around its circumference into the groove it is to
fit, which can be done without springing it over the piston as the
outside edge of the ring may be used to test the width of the groove
just as well as the inside edge. The ring should be a fair fit and while
free to move circumferentially there should be no appreciable up and
down motion. If the ring is a tight fit it should be laid edge down upon
the piece of emery cloth which is placed on the surface plate and
carefully rubbed down until it fits the groove it is to occupy. It is
advisable to fit each piston ring individually and to mark them in some
way to insure that they will be placed in the groove to which they are
fitted.
The repairman next turns his attention to fitting the ring in the
cylinder itself. The ring should be pushed into the cylinder at least
two inches up from the bottom and endeavor should be made to have the
lower edge of the ring parallel with the bottom of the cylinder. If the
ring is not of correct diameter, but is slightly larger than the
cylinder bore, this condition will be evident by the angular slots of
the rings being out of line or by difficulty in inserting the ring if it
is a lap joint form. If such is the case the ring is removed from the
cylinder and placed in the vise between soft metal jaw clips. Sufficient
metal is removed with a fine file from the edges of the ring at the slot
until the edges come into line and a slight space exists between them
when the ring is placed into the cylinder. It is important that this
space be left between the ends, for if this is not done when the ring
becomes heated the expansion of metal may cause the ends to abut and the
ring to jam in the cylinder.
[Illustration: Fig. 186.--Method of Removing Piston Rings, and Simple
Clamp to Facilitate Insertion of Rings in Cylinder.]
It is necessary to use more than ordinary caution in replacing the rings
on the piston because they are usually made of cast iron, a metal that
is very fragile and liable to break because of its brittleness. Special
care should be taken in replacing new rings as these members are more
apt to break than old ones. This is probably accounted for by the
heating action on used rings which tends to anneal the metal as well as
making it less springy. The bottom ring should be placed in position
first which is easily accomplished by springing the ring open enough to
pass on the piston and then sliding it into place in the lower groove
which on some types of engines is below the wrist pin, whereas in others
all grooves are above that member. The other members are put in by a
reversal of the process outlined at Fig. 186, A and B. It is not always
necessary to use the guiding strips of metal when replacing rings as it
is often possible, by putting the rings on the piston a little askew and
maneuvering them to pass the grooves without springing the ring into
them. The top ring should be the last one placed in position.
Before placing pistons in the cylinder one should make sure that the
slots in the piston rings are spaced equidistant on the piston, and if
pins are used to keep the ring from turning one should be careful to
make sure that these pins fit into their holes in the ring and that they
are not under the ring at any point. Practically all cylinders are
chamfered at the lower end to make insertion of piston rings easier. The
operation of putting on a cylinder casting over a piston really requires
two pairs of hands, one to manipulate the cylinder, the other person to
close the rings as they enter the cylinder. This may be done very easily
by a simple clamp member made of sheet brass or iron and used to close
the ring as indicated at Fig. 186, C. It is apparent that the clamp must
be adjusted to each individual ring and that the split portion of the
clamp must coincide with the split portion of the ring. The cylinder
should be well oiled before any attempt is made to install the pistons.
The engine should be run with more than the ordinary amount of lubricant
for several hours after new piston rings have been inserted. On first
starting the engine, one may be disappointed in that the compression is
even less than that obtained with the old rings. This condition will
soon be remedied as the rings become polished and adapt themselves to
the contour of the cylinder.
WRIST PIN WEAR
While wrist pins are usually made of very tough steel, case hardened
with the object of wearing out an easily renewable bronze bushing in the
upper end of the connecting rod rather than the wrist pin it sometimes
happens that these members will be worn so that even the replacement of
a new bushing in the connecting rod will not reduce the lost motion and
attendant noise due to a loose wrist pin. The only remedy is to fit new
wrist pins to the piston. Where the connecting rod is clamped to the
wrist pin and that member oscillates in the piston bosses the wear will
usually be indicated on bronze bushings which are pressed into the
piston bosses. These are easily renewed and after running a reamer
through them of the proper size no difficulty should be experienced in
replacing either the old or a new wrist pin depending upon the condition
of that member. If no bushings are provided, as in alloy pistons, the
bosses can sometimes be bored out and thin bushings inserted, though
this is not always possible. The alternative is to ream out the bosses
and upper end of rod a trifle larger after holes are trued up and fit
oversize wrist pins.
INSPECTION AND REFITTING OF ENGINE BEARINGS
While the engine is dismantled one has an excellent opportunity to
examine the various bearing points in the engine crank-case to ascertain
if any looseness exists due to depreciation of the bearing surfaces. As
will be evident, both main crank-shaft bearings and the lower end of the
connecting rods may be easily examined for deterioration. With the rods
in place, it is not difficult to feel the amount of lost motion by
grasping the connecting rod firmly with the hand and moving it up and
down. After the connecting rods have been removed and the propeller hub
taken off the crank-shaft to permit of ready handling, any looseness in
the main bearing may be detected by lifting up on either the front or
rear end of the crank-shaft and observing if there is any lost motion
between the shaft journal and the main bearing caps. It is not necessary
to take an engine entirely apart to examine the main bearings, as in
most forms these may be readily reached by removing the sump. The
symptoms of worn main bearings are not hard to identify. If an engine
knocks regardless of speed or spark-lever position, and the trouble is
not due to carbon deposits in the combustion chamber, one may reasonably
surmise that the main bearings have become loose or that lost motion may
exist at the connecting rod big ends, and possibly at the wrist pins.
The main journals of any well resigned engine are usually proportioned
with ample surface and will not wear unduly unless lubrication has been
neglected. The connecting rod bearings wear quicker than the main
bearings owing to being subjected to a greater unit stress, and it may
be necessary to take these up.
ADJUSTING MAIN BEARINGS
[Illustration: Fig. 187.--Tools and Processes Used in Refitting Engine
Bearings.]
When the bearings are not worn enough to require refitting the lost
motion can often be eliminated by removing one or more of the thin shims
or liners ordinarily used to separate the bearing caps from the seat.
These are shown at Fig. 187, A. Care must be taken that an even number
of shims of the same thickness are removed from each side of the
journal. If there is considerable lost motion after one or two shims
have been removed, it will be advisable to take out more shims and to
scrape the bearing to a fit before the bearing cap is tightened up. It
may be necessary to clean up the crank-shaft journals as these may be
scored due to not having received clean oil or having had bearings seize
upon them. It is not difficult to true up the crank-pins or main
journals if the score marks are not deep. A fine file and emery cloth
may be used, or a lapping tool such as depicted at Fig. 187, B. The
latter is preferable because the file and emery cloth will only tend to
smooth the surface while the lap will have the effect of restoring the
crank to proper contour.
A lapping tool may be easily made, as shown at B, the blocks being of
lead or hard wood. As the width of these are about half that of the
crank-pin the tool may be worked from side to side as it is rotated. An
abrasive paste composed of fine emery powder and oil is placed between
the blocks, and the blocks are firmly clamped to the crank-pin. As the
lead blocks bed down, the wing nut should be tightened to insure that
the abrasive will be held with some degree of pressure against the
shaft. A liberal supply of new abrading material is placed between the
lapping blocks and crank-shaft from time to time and the old mixture
cleaned off with gasoline. It is necessary to maintain a side to side
movement of the lapping tool in order to have the process affect the
whole width of the crank-pin equally. The lapping is continued until a
smooth surface is obtained. If a crank-pin is worn out of true to any
extent the only method of restoring it is to have it ground down to
proper circular form by a competent mechanic having the necessary
machine tools to carry on the work accurately. A crank-pin truing tool
that may be worked by hand is shown at Fig. 187, K.
After the crank-shaft is trued the next operation is to fit it to the
main bearings or rather to scrape these members to fit the shaft
journal. In order to bring the brasses closer together, it may be
necessary to remove a little metal from the edges of the caps to
compensate for the lost motion. A very simple way of doing this is shown
at Fig. 187, D. A piece of medium emery cloth is rested on the surface
plate and the box or brass is pushed back and forth over that member by
hand, the amount of pressure and rapidity of movement being determined
by the amount of metal it is necessary to remove. This is better than
filing, because the edges will be flat and there will be no tendency
for the bearing caps to rock when placed against the bearing seat. It is
important to take enough off the edges of the boxes to insure that they
will grip the crank tightly. The outer diameter must be checked with a
pair of calipers during this operation to make sure that the surfaces
remain parallel. Otherwise, the bearing brasses will only grip at one
end and with such insufficient support they will quickly work loose,
both in the bearing seat and bearing cap.
SCRAPING BRASSES TO FIT
To insure that the bearing brasses will be a good fit on the trued-up
crank-pins or crank-shaft journals, they must be scraped to fit the
various crank-shaft journals. The process of scraping, while a tedious
one, is not difficult, requiring only patience and some degree of care
to do a good job. The surface of the crank-pin is smeared with Prussian
blue pigment which is spread evenly over the entire surface. The
bearings are then clamped together in the usual manner with the proper
bolts, and the crank-shaft revolved several times to indicate the high
spots on the bearing cap. At the start of the process of scraping in,
the bearing may seat only at a few points as shown at Fig. 187, G.
Continued scraping will bring the bearing surface as indicated at H,
which is a considerable improvement, while the process may be considered
complete when the brass indicates a bearing all over as at I. The high
spots are indicated by blue, as where the shaft does not bear on the
bearing there is no color. The high spots are removed by means of a
scraping tool of the form shown at Fig. 187, F, which is easily made
from a worn-out file. These are forged to shape and ground hollow as
indicated in the section, and are kept properly sharpened by frequent
rubbing on an ordinary oil stone. To scrape properly, the edge of the
scraper must be very keen. The straight and curved half-round scrapers,
shown at M and N, are used for bearings. The three-cornered scraper,
outlined at O, is also used on curved surfaces, and is of value in
rounding off the sharp corners. The straight or curved half-round type
works well on soft-bearing metals, such as babbitt, or white brass, but
on yellow brass or bronze it cuts very slowly, and as soon as the edge
becomes dull considerable pressure is needed to remove any metal, this
calling for frequent sharpening.
When correcting errors on flat or curved surfaces by hand-scraping, it
is desirable, of course, to obtain an evenly spotted bearing with as
little scraping as possible. When the part to be scraped is first
applied to the surface-plate, or to a journal in the case of a bearing,
three or four "high" spots may be indicated by the marking material. The
time required to reduce these high spots and obtain a bearing that is
distributed over the entire surface depends largely upon the way the
scraping is started. If the first bearing marks indicate a decided rise
in the surface, much time can be saved by scraping larger areas than are
covered by the bearing marks; this is especially true of large shaft and
engine bearings, etc. An experienced workman will not only remove the
heavy marks, but also reduce a larger area; then, when the bearing is
tested again, the marks will generally be distributed somewhat. If the
heavy marks which usually appear at first are simply removed by light
scraping, these "point bearings" are gradually enlarged, but a much
longer time will be required to distribute them.
The number of times the bearing must be applied to the journal for
testing is important, especially when the box or bearing is large and
not easily handled. The time required to distribute the bearing marks
evenly depends largely upon one's judgment in "reading" these marks. In
the early stages of the scraping operation, the marks should be used
partly as a guide for showing the high areas, and instead of merely
scraping the marked spot the surface surrounding it should also be
reduced, unless it is evident that the unevenness is local. The idea
should be to obtain first a few large but generally distributed marks;
then an evenly and finely spotted surface can be produced quite easily.
In fitting brasses when these are of the removable type, two methods may
be used. The upper half of the engine base may be inverted on a suitable
bench or stand and the boxes fitted by placing the crank-shaft in
position, clamping down one bearing cap at a time and fitting each
bearing in succession until they bed equally. From that time on the
bearings should be fitted at the same time so the shaft will be parallel
with the bottom of the cylinders. Considerable time and handling of the
heavy crank-shaft may be saved if a preliminary fitting of the bearing
brasses is made by clamping them together with a carpenter's wood clamp
as shown at Fig. 187, J, and leaving the crank-shaft attached to the
bench as shown at C. The brasses are revolved around the crank-shaft
journal and are scraped to fit wherever high spots are indicated until
they begin to seat fairly. When the brasses assume a finished appearance
the final scraping should be carried on with all bearings in place and
revolving the crank-shaft to determine the area of the seating. When the
brasses are properly fitted they will not only show a full bearing
surface, but the shaft will not turn unduly hard if revolved with a
moderate amount of leverage.
Bearings of white metal or babbitt can be fitted tighter than those of
bronze, and care must be observed in supplying lubricant as considerably
more than the usual amount is needed until the bearings are run in by
several hours of test block work. Before the scraping process is started
it is well to chisel an oil groove in the bearing as shown at Fig. 187,
L. Grooves are very helpful in insuring uniform distribution of oil over
the entire width of bearing and at the same time act as reservoirs to
retain a supply of oil. The tool used is a round-nosed chisel, the
effort being made to cut the grooves of uniform depth and having smooth
sides. Care should be taken not to cut the grooves too deeply, as this
will seriously reduce the strength of the bearing bushing. The shape of
the groove ordinarily provided is clearly shown at Fig. 187, G, and it
will be observed that the grooves do not extend clear to the edge of the
bearing, but stop about a quarter of an inch from that point. The hole
through which the oil is supplied to the bearing is usually drilled in
such a way that it will communicate with the groove.
The tool shown at Fig. 187, K, is of recent development, and is known as
a "crank-shaft equalizer." This is a hand-operated turning tool,
carrying cutters which are intended to smooth down scored crank-pins
without using a lathe. The feed may be adjusted by suitable screws and
the device may be fitted to crank-pins and shaft-journals of different
diameters by other adjusting screws. This device is not hard to operate,
being merely clamped around the crank-shaft in the same manner as the
lapping tool previously described, and after it has been properly
adjusted it is turned around by the levers provided for the purpose, the
continuous rotary motion removing the metal just as a lathe tool would.
FITTING CONNECTING RODS
In the marine type rod, which is the form generally used in airplane
engines, one or two bolts are employed at each side and the cap must be
removed entirely before the bearing can be taken off of the crank-pin.
The tightness of the brasses around the crank-pin can never be
determined solely by the adjustment of the bolts, as while it is
important that these should be drawn up as tightly as possible, the
bearing should fit the shaft without undue binding, even if the brasses
must be scraped to insure a proper fit. As is true of the main bearings,
the marine form of connecting rod in some engines has a number of liners
or shims interposed between the top and lower portions of the rod end,
and these may be reduced in number when necessary to bring the brasses
closer together. The general tendency in airplane engines is to
eliminate shims in either the main or connecting rod bearings, and when
wear is noticed the boxes or liners are removed and new ones supplied.
The brasses are held in the connecting rod and cap by brass rivets and
are generally attached in the main bearing by small brass machine
screws. The form of box generally favored is a brass sand casting rich
in copper to secure good heat conductivity which forms a backing for a
thin layer of white brass, babbitt or similar anti-friction metal.
[Illustration: Fig. 188.--Showing Points to Observe When Fitting
Connecting Rod Brasses.]
In fitting new brasses there are two conditions to be avoided, these
being outlined at Fig. 188, B and C. In the case shown at C the light
edges of the bushings are in contact, but the connecting rod and its cap
do not meet. When the retaining nuts are tightened the entire strain is
taken on the comparatively small area of the edges of the bushings which
are not strong enough to withstand the strains existing and which
flatten out quickly, permitting the bearing to run loose. In the example
outlined at B the edges of the brasses do not touch when the connecting
rod cap is drawn in place. This is not good practice, because the
brasses soon become loose in their retaining member. In the case
outlined it is necessary to file off the faces of the rod and cap until
these meet, and to insure contact of the edges of the brasses as well.
In event of the brasses coming together before the cap and rod make
contact, as shown at C, the bearing halves should be reduced at the
edges until both the caps and brasses meet against each other or the
surfaces of the liners as shown at A.
SPRUNG CAM-SHAFT
If the cam-shaft is sprung or twisted it will alter the valve timing to
such an extent that the smoothness of operation of the engine will be
materially affected. If this condition is suspected the cam-shaft may be
swung on lathe centers and turned to see if it runs out and can be
straightened in any of the usual form of shaft-straightening machines.
The shaft may be twisted without being sprung. This can only be
determined by supporting one end of the shaft in an index head and the
other end on a milling machine center. The cams are then checked to see
that they are separated by the proper degree of angularity. This process
is one that requires a thorough knowledge of the valve timing of the
engine in question, and is best done at the factory where the engine was
made. The timing gears should also be examined to see if the teeth are
worn enough so that considerable back lash or lost motion exists between
them. This is especially important where worm or spiral gears are used.
A worn timing gear not only produces noise, but it will cause the time
of opening and closing of the engine valves to vary materially.
PRECAUTIONS IN REASSEMBLING PARTS
When all of the essential components of a power plant have been
carefully looked over and cleaned and all defects eliminated, either by
adjustment or replacement of worn portions, the motor should be
reassembled, taking care to have the parts occupy just the same
relative positions they did before the motor was dismantled. As each
part is added to the assemblage care should be taken to insure adequate
lubrication of all new points of bearing by squirting liberal quantities
of cylinder oil upon them with a hand oil can or syringe provided for
the purpose. In adjusting the crank-shaft bearings, tighten them one at
a time and revolve the shafts each time one of the bearing caps is set
up to insure that the newly adjusted bearing does not have undue
friction. All retaining keys and pins must be positively placed and it
is good practice to cover such a part with lubricant before replacing it
because it will not only drive in easier, but the part may be removed
more easily if necessary at some future time. If not oiled, rust
collects around it.
When a piece is held by more than one bolt or screw, especially if it is
a casting of brittle material such as cast iron or aluminum, the
fastening bolts should be tightened uniformly. If one bolt is tightened
more than the rest it is liable to spring the casting enough to break
it. Spring washers, check nuts, split pins or other locking means should
always be provided, especially on parts which are in motion or subjected
to heavy loads.
Before placing the cylinder over the piston it is imperative that the
slots in the piston rings are spaced equidistant and that the piston is
copiously oiled before the cylinder is slipped over it. When
reassembling the inlet and exhaust manifolds it is well to use only
perfect packings or gaskets and to avoid the use of those that seem to
have hardened up or flattened out too much in service. If it is
necessary to use new gaskets it is imperative to employ these at all
joints on a manifold, because if old and new gaskets are used together
the new ones are apt to keep the manifold from bedding properly upon the
used ones. It is well to coat the threads of all bolts and screws
subjected to heat, such as cylinder head and exhaust manifold retaining
bolts, with a mixture of graphite and oil. Those that enter the water
jacket should be covered with white or red lead or pipe thread
compound. Gaskets will hold better if coated with shellac before the
manifold or other parts are placed over them. The shellac fills any
irregularities in the joint and assists materially in preventing leakage
after the joint is made up and the coating has a chance to set.
Before assembling on the shaft, it is necessary to fit the bearings by
scraping, the same instructions given for restoring the contour of the
main bearings applying just as well in this case. It is apparent that if
the crank-pins are not round no amount of scraping will insure a true
bearing. A point to observe is to make sure that the heads of the bolts
are imbedded solidly in their proper position, and that they are not
raised by any burrs or particles of dirt under the head which will
flatten out after the engine has been run for a time and allow the bolts
to slack off. Similarly, care should be taken that there is no foreign
matter under the brasses and the box in which they seat. To guard
against this the bolts should be struck with a hammer several times
after they are tightened up, and the connecting rod can be hit sharply
several times under the cap with a wooden mallet or lead hammer. It is
important to pin the brasses in place to prevent movement, as
lubrication may be interfered with if the bushing turns round and breaks
the correct register between the oil hole in the cap and brasses.
Care should be taken in screwing on the retaining nuts to insure that
they will remain in place and not slack off. Spring washers should not
be used on either connecting rod ends or main bearing nuts, because
these sometimes snap in two pieces and leave the nut slack. The best
method of locking is to use well-fitting split pins and castellated
nuts.
TESTING BEARING PARALLELISM
It is not possible to give other than general directions regarding the
proper degree of tightening for a connecting rod bearing, but as a guide
to correct adjustment it may be said that if the connecting rod cap is
tightened sufficiently so the connecting rod will just about fall over
from a vertical position due to the piston weight when the bolts are
fully tightened up, the adjustment will be nearly correct. As previously
stated, babbitt or white metal bearings can be set up more tightly than
bronze, as the metal is softer and any high spots will soon be leveled
down with the running of the engine. It is important that care be taken
to preserve parallelism of the wrist-pins and crank-shafts while
scraping in bearings. This can be determined in two ways. That shown at
Fig. 189, A, is used when the parts are not in the engine assembly and
when the connecting rod bearing is being fitted to a mandrel or arbor
the same size as the crank-pin. The arbor, which is finished very smooth
and of uniform diameter, is placed in two V blocks, which in turn are
supported by a level surface plate. An adjustable height gauge may be
tried, first at one side of the wrist-pin which is placed at the upper
end of the connecting rod, then at the other, and any variation will be
easily determined by the degree of tilting of the rod. This test may be
made with the wrist-pin alone, or if the piston is in place, a straight
edge or spirit level may be employed. The spirit level will readily show
any inclination while the straight edge is used in connection with the
height gauge as indicated. Of course, the surface plate must be
absolutely level when tests are made.
When the connecting rods are being fitted with the crank-shaft in place
in crank-case, and that member secured in the frame, a steel square may
be used as it is reasonable to assume that the wrist-pin, and
consequently the piston it carries, should observe a true relation with
the top of the engine base. If the piston side is at right angles with
the top of the engine base it is reasonable to assume that the wrist-pin
and crank-pin are parallel. If the piston is canted to one side or the
other, it will indicate that the brasses have been scraped tapering,
which would mean considerable heating and undue friction if the piston
is installed in the cylinder on account of the pressure against one
portion of the cylinder wall. If the degree of canting is not too great,
the connecting rods may be sprung very slightly to straighten up the
piston, but this is a makeshift that is not advised. The height gauge
method shown above may be used instead of the steel square, if desired,
because the top of the crank-case is planed or milled true and should be
parallel with the center line of the crank-shaft.
[Illustration: Fig. 189.--Methods of Testing to Insure Parallelism of
Bearings After Fitting.]
CAM-SHAFTS AND TIMING GEARS
Knocking sounds are also evident if the cam-shaft is loose in its
bearings, and also if the cams or timing gears are loose on the shaft.
The cam-shaft is usually supported by solid bearings of the removable
bushing type, having no compensation for depreciation. If these bearings
wear the only remedy is replacement with new ones. In the older makes of
cars it was general practice to machine the cams separately and to
secure these to the cam-shaft by means of taper pins or keys. These
members sometimes loosened and caused noise. In the event of the cams
being loose, care should be taken to use new keys or taper pins, as the
case may be. If the fastening used was a pin, the hole through the
cam-shaft will invariably be slightly oval from wear. In order to insure
a tight job, the holes in cam and shaft must be reamed with the next
larger size of standard taper reamer and a larger pin driven in. Another
point to watch is the method of retaining the cam-shaft gear in place.
On some engines the gear is fastened to a flange on the cam-shaft by
retaining screws. These are not apt to become loose, but where reliance
is placed on a key the cam-shaft gear may often be loose on its
supporting member. The only remedy is to enlarge the key slot in both
gear and shaft and to fit a larger retaining key.