COMPRESSION TROUBLES
A leak in the combustion space will reduce the compression by permitting the fresh mixture to escape during the compression stroke, and will also give an escape for the pressure resulting from the combustion, the pressure on the piston being reduced in consequence. The most frequent cause of leakage is worn valves, the intense heat tending to warp the disks and to roughen their surfaces, the exhaust valve being especially liable to this as it is surrounded by the hot gases during the time that it is open. A badly fitting spark plug, igniter stem, relief cock, or other opening into the combustion space will produce the same result. Whenever possible, these parts should be fitted with copper asbestos washers, which are soft enough to squeeze into the inequalities of the surface, at the same time resisting the heat and pressure.
When piston rings or cylinder walls are cut or scratched by running without oil, the pressure will escape into the crank case, which, while warm when the engine is running properly, will be heated to a very noticeable degree under these circumstances. The only remedy is the use of new piston rings and the reboring of the cylinder. Piston rings, being of cast iron, are brittle, and must be handled carefully. To place them in position in their grooves, thin strips of steel, like pieces of a hack-saw blade, should be bound to the piston, covering all but the lower groove. The ring for this groove can then be slipped on, and when it is in position the steel strips moved upward to expose the upper grooves, one at a time.
Piston rings are usually prevented from revolving around the piston by pins driven into the piston between their ends; if these are not provided, the turning of the rings will bring their ends into line, and the pressure will leak through them. The appearance of the rings is the best indication as to whether they are permitting leakage; if they are tight, they will be smooth and polished all around, but if there is leakage, they will be streaked with black carbon deposit.
Other losses of compression may be due to a cracked piston, cylinder head or wall, the former being indicated by a hot crank case and the latter by the presence of water in the cylinder and crank case.