or four long lever arms, the latter being pulled round by a man stationed in the pit during the valve setting process. The rollers cause the driving wheels to revolve, only a small portion of a revolution being necessary at one movement during valve-setting. The wheels must be turned in the proper directions when setting the valves for forward or back gear.
Setting the Valves. Before the eccentric rods are coupled up to the expansion links, the points at which the valve closes the ports to steam are marked upon the valve spindles. A very thin piece of tin A₁ (Fig. 43–I) is held in the front port, and the valve with valve rod is moved carefully against it. The valve spindle S is then marked at a₁ outside the steam chest by means of the trammel T, the straight pointed end of which is placed in a centre “pop” made in the back of the steam chest cover, whilst the right-angled end scribes the mark a₁ on the valve spindle. A small “pop” is made with a sharp punch to show the position of this mark. Similarly the point a₂ at which the valve closes the port at A₂ at the other end (Fig. 43–II), is marked, by transferring the tin to the latter port and pushing the valve in the opposite direction against it. The two “pops” a₁ and a₂ on the spindle show the positions of the latter when the steam is cut off, since these are the points at which the valve closes the ports.
The eccentric rods are then coupled up, and the “dead centres,” i.e., the exact points at
