The second and more common cause of seizing or freezing of the piston is lack of cooling water. This may occur both while the engine is in operation and after it is shut down. The former is evidenced by the loss of power and decrease in the engine speed and usually happens on a change from light to heavy load. The average operator understands that on full load the quantity of cooling water required is greater than on the lower loads. As the load comes on, the usual practice is to increase the flow of the cooling water. This chills the cylinder liner and causes it to contract before the hot piston experiences any effect from this additional cooling medium. This contraction lessens the working clearance; the piston becomes hotter and ultimately grips the liner walls. The remedy is to decrease the amount of cooling water as the heavy load comes on; then, after the cylinder liner warms up, the flow of water can be gradually increased while the discharge temperature is kept fairly constant.
There are, also, occasions when the seizing can be attributed to a hot piston pin, and ordinarily this is noticeable on starting the engine. After an engine has been in operation for some minutes, the heat absorbed by the piston is equaled by the heat given off by the piston to the cylinder walls, etc. The heat contained in the piston when the engine is shut down is large. If the flow of cooling water is discontinued at once, this heat is slowly radiated and a great part is absorbed by the piston pin. The pin elongates as a result of this heat absorption. The end thrust of the pin produces a change in the shape of the piston walls; especially is this true when the walls are not strongly ribbed. The deformation of the piston occurs along the thinnest sections, which commonly are at the junction of the pin bosses with the piston walls. These ridges or deformations produce severe cutting of the cylinder walls. Many liners are ruined by this action. When this misfortune is experienced, the sole relief other than piston replacement is the filing of the high spots until the piston is again cylindrical. The surface can then be dressed by emery cloth. Filing will also smoothen the scored cylinder walls. A quick and efficacious repair can be accom plished by first dressing down the rough places with a fine emery wheel, say 80J grade, held in the hand, finishing with a file and emery cloth. To some this probably seems a radical treatment, but extensive experience on many scored pistons and cylinders tends to prove that this is an effective way to rectify the damage. If the engineer is careful in using the emery cloth, the cylinder and piston can be made as smooth as when new.
Piston seizing is at times in evidence when turning the engine over at the beginning of a run. If, at the close of the last run.
the cooling water was shut off too early, the heat in the piston head may evaporate all the lubricant on the piston pin. As the dry bronze bearing absorbs the heat, it cannot expand outward because of the greater mass of the connecting-rod end. The bearing closes in on the pin, which is also expanding; this action results in a gripping of the pin that is not completely loosened even after the parts have cooled. On starting, this wedged bear ing restrains the motion of the connecting-rod, and the engineer calls it a seized piston, though it actually is a case of a seized piston pin.
Piston Wear.—The wear of the piston is due to lack of lubrica tion, deformation and the natural abrasive action that results when two surfaces rub together. The lack of lubrication can be attributed to the carelessness of the operator. No matter what manner of oiling system is used, the engineer is never blameless when it fails so far as to wear or cut the piston.
The natural abrasive action requires years before the wear assumes such proportions as to necessitate a replacement. A piston should last from four to eight years dependent on the hours of service and on the degree of intelligent care it receives. It is impossible to set a hard and fast rule as to the clearance that can exist before replacement is imperative. A vertical trunk or horizontal crosshead type piston should have a clearance of around .007 inch, while the horizontal trunk piston should have still less. In operation, if the rings are in good shape, a piston will hold the compression quite satisfactorily if these values are doubled. With the clearance question, as with many Diesel problems, the engineer must allow the engine's performance to guide his actions.
Piston Rings.—The customary designs of rings have lapped ends and are constrained from shifting in the groove by dowels.
Some builders fasten the ring ends together with a pin. It is doubtful whether this serves any useful purpose. In turning up new rings, the casting should be made from gray cast iron free from scrap. The outside of the ring is machined to size. Then the casting is chucked inch out of center and the inside turned, the ring next being cut off. This produces a ring of a varying thickness and gives an almost uniform pressure entirely around its circumference. The outside diameter of the ring should be 716 inch greater than the cylinder bore of the engine. In cutting the lap, the clearance is best made about inch, while the lip may be as much as 1 inch. The lap can be drilled and then cut out with a hack-saw, as shown in Fig. 58. The edges of the ring are left square and should never be rounded as is practiced by many engineers.