STRIKING POINTS AND CLEARANCE.
Before putting up main rods, the striking points of the pistons should be located and marked on the guides. Then, when the rods are put up, the clearance should be divided equally between the two ends. The identification of these points is of greater interest to the engineer who is running the engine than to any other person; for upon their correctness the success of his running may, to some extent, depend. An engine may go out with the clearance badly divided, and run all right for a few days, and the driving of a key may then cause the piston to strike the head. A forcible instance of this kind once came under my observation. A careless machinist, in working on main-rod brasses, had mixed the liners, and shortened the rod, till the piston began to touch the back head. When the engine was working light, there was just a slight jar; but, when the load was heavy, the jar became a distinct pound. The engineer could not locate the knock, and was disposed to think it was in the driving-box. One day that he slipped the engine badly, steam began to issue from the back cylinder-head, which was cracked by a blow from the piston. The cause of the pound was then discovered.