Page:Steamlocomotivec00ahrorich.djvu/112

98 the cylinder, it would “bump” the cylinder covers. The engine is so designed that this does not take place, and a space of about ¼ to ⅜ in., known as the “clearance,” is allowed between the piston and the cylinder cover at each end of the stroke. Since the movement of the piston and of the crosshead connected to the piston rod are exactly the same, all measurements of piston stroke may be taken from the crosshead relatively to the slide bars. The “bumping marks” are scribed on the slide bars at each end before the connecting rod is put up, by pushing the piston by hand to each end as far as it will go until it strikes the cylinder covers, and marking the positions of one end of the crosshead slide block on the bars. When the connecting rods and motion have been put up, and the wheels revolved as described below, the terminal positions of the slide blocks should be distant from the bumping marks at each end by the amount of the clearance.

To set the valves it is found more convenient to turn the wheels by means of the following simple apparatus than to move the engine backwards and forwards on a length of line outside the shop. Two brackets are fixed beside the rails, one on each side of the pit over which the engine is built. These brackets have bearings in which rollers are fitted, and the engine is lifted slightly so that the circumferences of the driving wheels rest upon the rollers. The rollers are revolved by means of a shaft which lies across the pit, the shaft being provided with a ratchet