Piston ring leaks are usually caused by the rings sticking in their grooves through the formation of carbon. To test for piston ring leaks, pour a half pint of cylinder oil into each cylinder, and crank the engine slowly. The oil will form a seal around the pistons, and if compression is then improved, the rings are shown to be at fault. To free the rings, pour a few tablespoonfuls of kerosene into each cylinder, and spread it by giving the engine a few turns; after standing for an hour or so the carbon should be sufficiently softened to free the rings. If the leakage of compression is due to the rings being worn and loose in their grooves, they must be replaced. The most usual cause of compression loss is leaking valves. With its continual pounding against its seat, and the heat to which it is exposed, a valve and its seat will become rough and pitted, and will leak; when in this condition the valve must be ground. A valve is ground by spreading grinding compound on the seat, and turning the valve against it. This requires the valve spring to be taken off; the exact method of doing this depends on how these parts are made. If the valves are in a removable cylinder head, valve grinding is most easily done by taking the cylinder head to a bench. In many designs the valve seats are part of the cylinder casting, and the job is done on the tractor. In grinding a valve the valve is not turned around in one direction only, for this would cut grooves in the valve and seat. To obtain smooth surfaces the valve should be given part of a turn in one direction, and then turned equally in the other direction; after every few turns the valve should be lifted and dropped to another position on the seat. In this way the grinding is made even all around. [Illustration: FIG. 89.—GRINDING VALVE IN ENGINE WITH FIXED HEAD] 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.