From time to time the valve disk and seat should be cleaned off and examined to see whether they are smooth and free from pits and scores. If they appear to be, make marks around the valve disk with a lead pencil, replace the valve, and give it a complete turn. If this wipes off the pencil marks all around the valve, the grinding is complete, and the valve may be replaced with its spring and spring retainer. It is not necessary to grind until the entire thickness of the valve disk and seat are smooth; a narrow band all around will make the valve tight. After grinding, and before replacing the valve, all traces of the grinding compound should be wiped off, and great care taken that none of it gets into the cylinder, valve stem guide, or other working part. [Illustration: FIG. 90.—GRINDING VALVE IN DETACHABLE HEAD] On an engine with a removable head containing the valves, the head may be taken to a work bench, which makes grinding easier. This is illustrated in Figure 90. On an engine in which the valve and its seat may be taken out, the seat may be clamped in a vise, as shown in Figure 91. With valves of either of these types, the grinding may be tested by turning the head or the seat so that the disk is down, and pouring in gasoline. If the valve is not tight, the gasoline will leak through, and grinding must be continued. [Illustration: FIG. 91.—GRINDING VALVE IN DETACHABLE SEAT] When a valve seat is very badly worn it must be redressed, which is done with a cutting tool to be obtained from the maker of the tractor, and illustrated in Figure 92. This has a stem fitting the valve stem guide which centers the tool and assures a true cut. If a seat is so worn as to need redressing, the valve will be in such bad condition that it must be discarded and a new one used. This must be ground in before the engine is run. 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. [Illustration: 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. [Illustration: 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