Fractured Piston Heads.—The cracks that develop in the piston head can be placed in two classes; namely, those which take a circular form and appear around the base of the concave portion of the head, and those fractures which are radial in direction. The former are caused by heat strains with a conse quent breathing action at the base of the cone. These fractures seldom prove serious, and it is possible to continue to operate the engine without piston replacement. At times these fractures appear in conjunction with cracks on the interior side of the pis ton head, across the reinforcing ribs. In such cases the entire head may give way.
The really serious fractures are those that develop radially in the head. Often these cracks extend across the head some 6 to 10 inches, Fig. 59. The danger lies in the now non-rigid head allowing the piston to distort, scoring the cylinder walls.
When the fracture is only a few inches in length it can be re paired by "sewing." A hole should be drilled at each end of the crack to prevent any further development. A series of N-inch holes are drilled and tapped along the line of the fracture. Into these holes threaded brass plugs should be inserted and cut smooth with the surface. Between these plugs a second row is inserted, lapping over the first plugs. This entire line of plugs is then hammered smooth; see Fig. 60. This "sewing" has been practised with success on pistons as large as 18 inches in diameter.
Piston Ring Troubles.—Beyond an occasional broken ring the only difficulty that the operator will experience is the gum ming of the rings in the grooves. The gumming may be pro duced either by an excessive amount of fuel oil which remains in the cylinder in an unconsumed condition or by an overabundance of lubricating oil. The solution of the latter trouble is simple; all that need be done is the reduction of the quantity supplied. If the oiling is accomplished by a mechanical pump, the control can be readjusted. If the engine has splash lubrication, the oil level can be lowered, thereby preventing undue throwing of the oil emulsion on to the cylinder walls. Where the trouble is traceable to excessive fuel oil, the problem is not as simple of solution. It is apparent that, in such cases, the difficulty lies in the fuel valve. The timing may be incorrect, or the air pressure too low or too high, or the atomizer disks or cones may be out of order. It becomes necessary to try various experiments until the proper remedy is obtained.
The rings can be loosened by soaking the entire piston in kerosene or lye water, after which the rings can be pried off. In removing the gum and carbon a copper or brass scraper should be used. It is not advisable to clean with emery paper as long as the piston is perfectly smooth. Each time the engine is laid
up for a few hours a small quantity of kerosene should be in jected into the cylinder through the air admission valve. This kerosene will remove any carbon on the piston and rings that is in the process of formation. The exhaust valve, in such event, is best blocked open to allow the vapor to escape.
Grinding Taper Piston number of engines have ,piston pins with taper ends. The tapered ends fit into ground seats in the piston bosses. The ground seats gradually pounds out of round, forcing the engineer to regrind the pin to new bearings. To accomplish this with the minimum of trouble the piston can be placed on two 8X8 in. timbers, as shown in Fig. 61. A discarded valve spring, if set under the piston immediately below the pin, will keep the pin raised from the seats in the bosses unless the pin is pressed downward. Emery paste is coated over the two pin ends, and while the pin is forced into the bosses it is rotated by a pin wrench, as outlined in the sketch. Removal of the downward pressure allows the spring to raise the pin. This action serves to distribute the grinding paste over the entire seat, preventing the seat from being ground hollow. The pin grinding is fully as important as valve grinding although it is done very carelessly in many plants.
In reassembling the piston, pin and rod it is not enough to sim ply push the pin into the bosses and tighten up on the lock-nut. The pin must be driven in with a sledge and copper mallet with as great a blow as an able-bodied man can deliver. The tim orous feeling many engineers have about this driving process is entirely without foundation; there is no danger of fracturing the piston.
Worn Piston Bosses.—On old engines the bosses sometimes wear so oval as to allow the pin to fit loosely. It is impossible to secure a good seat if this wear is great. A partial remedy for this condition is effected by peening the bosses on the outside, followed up by regrinding.
Emergency Piston Pin.—When a pin has scored badly or developed a fracture, it occasionally becomes necessary to make an emergency pin. Without question the manufacturer is the proper party to furnish this new part. But where the engine is urgently required, the engineer cannot wait on the slow delivery that is so prevalent. A makeshift pin can be turned from cold rolled shafting. It should be machined to size and then ease-hardened with bone-black or cyanide of potassium. After remaining in the fire for twelve hours, the pin should be cooled off. As a rule, the pin expands but slightly in the hard ening process. The exact dimensions can be secured by lapping with emery paste, although it is a laborious procedure.