The best tool for valve grinding is a carpenter’s brace with a screw driver blade fitting the slot in the valve, as shown in [Figure 89]. This drawing illustrates a cylinder with a fixed head; the valve is reached by unscrewing the plug from the opening directly above it. When grinding valves in an engine of this design the opening between the valve pocket and the combustion space should be plugged with a rag or waste to prevent the grinding compound from getting into the cylinder.
With the valve grinding tool in position, swing the handle back and forth ten or twelve times; then lift the valve, place it in a new position, and repeat. The valve is lifted most easily by a light spring placed under the valve disk, as shown in [Figure 89].
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.
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.
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.