MARKING THE VALVE-STEM.
Fig. 17.
In ordinary practice, valves are set with the steam-chest cover down, and the position of the valve on the seat is identified by marks on the valve-stem. Before the cover is put down, the valve is placed as in [Fig. 17], just beginning to open the forward steam-port; a thin piece of tin being generally used to gauge the opening. When the valve stands in this position, a tram is extended from a center punch-mark c, on the stuffing-box, straight along the valve-stem as far as it will reach; and the point, here located at a, is marked. The valve is then moved forward till it begins to uncover the back port, when another measurement is made with the tram, which locates the point b on the valve-stem. Whatever position the valve may stand on, it may now be identified by the tram. When the tram cuts the space half way between a and b, the valve stands in the middle of the seat.
Some machinists do not believe in tramming from the stuffing-box, as the point is liable to be moved in tightening down the steam-chest cover. These generally measure from a point on the cylinder casting, but that practice has its drawbacks.