The Knocks or Pounds of steam engines are frequently caused by
lost motion in the crank and wrist-pin boxes, valve rod and valve,
cross-head and link; looseness of the piston rod, pillow block or main
bearings, follower plate, or eccentrics; the slide valve having not
enough or too much lead, the exhaust being cut off too soon or too
late, shoulders being worn in each end of the cylinder by the packing
rings not traveling over the counter-bore at the end of each stroke;
or shoulders may be worn in the guides by the cross-head slides, or
they may not be adjusted properly to fit the guides; boiler may foam,
causing the water to be drawn over in cylinder; the piston rings may
leak, thus causing cushioning; and as the crank approaches the centers,
steam occupies the space between the cylinder head and piston, causing
a tremendous strain upon the engine; the piston rod being packed too
tight, the boxes and pins being worn flat or oval; the key in driving
pulley may be loose.