It sometimes happens that part of the babbitt cracks and drags around the pin. This results in heating and a badly scored bear ing. If the trouble is local, the rough spots can be smoothed with a scraper, and the bearing can then be placed in service again. In all instances where big-end bearings become so hot that the babbitt is thrown, the engine should not be stopped immedi ately; rather, the load should be taken off and the engine turned over very slowly, with the particular cut out. In re babbitting rod bearings the same method as described for main bearings can be followed. The babbitt should always be cut on a bevel at the junction of the two halves. The oil grooves should not extend to the bearing edges, and, when a pressure oiling system is used, the oil grooves should be eliminated since they allow the oil to escape too rapidly.
Side-play is of frequent occurrence in Diesel operation. The best method is to tin the bearing sides and run a collar of babbitt around the bore. This collar must be turned square with the pin. It is not necessary to cover the entire side; consequently, the collar can be machined parallel with the side of the big end. Another method is the employment of sheet-steel washers between the web and the big-end sides. Inspection of the connecting-rod
bearings should be performed at least every three months.
Along with the wear of the big-end bearing occurs the wear of the pin. In old engines, which have seen several years of service, the pins may become flattened on one side. This can be corrected by filing and lapping, but it is a task requiring great care and patience. It is, ordinarily, not difficult to detect a worn rod bearing. The engine will emit a thump or pound both on the in and out strokes.
Pin Clearance.—To estimate the amount of clearance neces sary between bearing and pin, an excellent scheme is to "jump" the rod with a bar. If a slight movement can be felt, the big end has ample clearance. "Jumping" the piston allows one to judge the piston-pin clearance. Another and better method is to tighten up the piston-pin bearing until the connecting-rod can be barely swung back and forth when the piston is suspended by a chain hoist. If the movement is free, there is too much clearance. If a man cannot swing the rod without tilting or moving the piston, the bearing is too tight.