All About Engines

The Modern Reciprocating Engine 107 of a layer of water on the top, and Watt, owing to his cylinders differing from a circular form by three- eighths of an inch or more, was obliged to use a soft packing to fill up the space. These methods soon gave way to “ metal to metal ” joints. There is bound to be some wear, and as the replacement of a whole cylinder would be a costly affair, it is pro- vided with a “ liner,” which is cast separately and then pressed into the cylinder, which it fits only at the ends. It is of a harder, closer-grained iron than the cylinder itself, and it is very accurately bored to Fig. 59.'—Piston for email engine Fig. 60.—Piston with junk ring for large engine receive the piston. The space between the liner and the cylinder walls forms a steam jacket which pre- vents condensation of the steam used to drive the engine. The piston is usually of cast-iron, though it may be of cast or pressed steel. In small sizes it is in one piece, with two or three grooves cut in the curved face. Cast-iron rings, turned so as to be slightly larger than the bore of the cylinder or liner, are then sawn across diagonally at one point, and “ sprung ” into the grooves, as in Fig. 59. In large sizes the rings are too stiff to be sprung on, and the piston has to be made with a single wide