Grinding a valve lowers it in its seat, and usually makes it necessary to readjust the push rod. When an engine is cold there is a space of about ¹/₃₂ inch somewhere between the cam and the valve stem; in [Figure 93], this space is shown to be between the valve stem and the rocker arm. As the engine heats up the valve stem lengthens, and this space permits it to do so.
Fig. 92.—Valve Seat Cutter
If the space is too small, the stem will come against the rocker arm or the push rod, and the valve will be held off its seat, causing a compression leak. If the space is too great, the valve will open too late and close too early. The space must therefore be carefully adjusted, and this is arranged for on practically all makes of tractor engines.
Fig. 93.—“Holt” Valve Arrangement
One-thirty-second of an inch is the thickness of a 10-cent piece; it should just be possible to slip a slightly worn dime into the space when the engine is cold.
VALVE TIMING
By timing the valves is meant the setting of the cam shaft in such a position that the valves are opened at the correct point in the stroke. It is necessary to time the valves only when the cam shaft has been taken out and must be replaced. The principle of valve timing should be understood, however, in order to be able to tell whether an engine is timed correctly.
It will usually be found that the face of the flywheel bears letters and figures that are indicators of the timing of the valves. This arrangement on the E-B engines is shown in [Figure 94]. Two lines are cut in the face of the flywheel, one marked ex. cl. 1-4, which means exhaust valve closes, cylinders 1 and 4, and the other marked center 1-4, to indicate that the pistons in those cylinders are on center. A straight-edge is held against the finished surface of the housing and the crank shaft is turned to bring one of the marks in line with it; at that point the valves or pistons are as indicated by the lettering.