SPARK PLUGS AND CARE
Some definite knowledge of spark plug construction quality, and care,
will be found very useful to the average motorist in purchasing new
plugs, and keeping those in present use, in good condition. A good
plug properly constructed should outlast the life of the motor. When
purchasing new plugs, first examine the old plug and get one of the
same length. This is very important as spark plugs are made in as many
different lengths as required by high and low compression motors. High
compression motors have a small low walled combustion chamber, while
low compression motors usually have a spacious high wall chamber and
require a longer plug, whereas if the long plug is used in the high
compression motor it may be put out of commission by the ascending
piston. Next determine the size of the plug and the gauge of the
thread. The majority of motors use the ³⁄₄ inch plug, with the S. A.
E. thread, while a few still use the A. L. A. M. thread which is much
finer gauged. Another point to be remembered is that it is an unwise
expenditure to purchase cheap plugs because the intense heat and
pressure that they are subjected to and required to stand, demands that
they be made of the highest quality of material and workmanship.
[Illustration: Fig. 104. Spark Plug]
Fig. 104 shows the sectional construction of a spark plug costing
from one dollar to one dollar and fifty cents. No. 1, the terminal,
is designed to fit all connections. No. 2 nut which holds electroids
firmly in place. No. 3 represents round edged shoulders which prevent
the plug from short circuiting on the outside. No. 4 is a heavy
electroid which will not break or burn. No. 5 is an extra heavy
insulator which insures a good spark in case the outer porcelain
insulator becomes broken or cracked. No. 6 is a bushing which holds
the insulator firmly in place from the top. No. 7 is a high compression
washer which allows for upward expansion and makes an even seat
for the bushing which holds the insulator in position. No. 8 is a
massive porcelain insulator designed to withstand a high temperature
without cracking. No. 9 is a copper asbestos washer that allows for
the downward expansion of the insulator. No. 10 is the shell casting
which holds and protects the insulator. No. 11 are rounded corners
which will allow the plug to be screwed down flush without coming into
contact with the curved walls of the cup containers. No. 12 is a high
compression washer which prevents all leakage. No. 13 shows elastic
cement which strengthens the lower construction of the insulator and
prevents the compression from escaping through the center of the
insulator. No. 14 is a hardened polished steel tipped electroid. No.
15 is a bent polished steel electroid dipped on each side of the spark
in order to prevent oil from running down from the shell casting and
closing the spark gap. No. 16 represents an extended center electroid
which prevents any oil that may have lodged on it from stopping at the
spark gap.
=Spark Plug Cleaning.=--To insure a smooth running motor and a good
spark, the spark plugs should be cleaned at thirty day intervals. It
is not always necessary to disassemble them at this time as the carbon
usually collects and bakes on the metal casting shell and can be
removed by running a thin knife blade or finger nail file around the
inner surface. However, when the insulator becomes pitted or carbon
burnt the plug should be disassembled and the insulator wiped clean
with a cloth dampened in kerosene. Never immerse the insulator in
kerosene, as this will loosen the cement around the center electroid
and cause the plug to leak compression. The shell may be immersed. It
is then wiped dry and the inside surfaces scraped or rubbed with a
piece of sand or emery paper to dislodge the carbon pits. After all
parts have been thoroughly dried the plug is reassembled, using new
washers.