The exhaust valves should be lubricated with Gas Engine Cylinder Oil, never with common machine oil on account of gumming and sticking, or with gas engine cylinder oil thickened with =FLAKE GRAPHITE=. Powdered graphite may be used with success without the addition of oil, but oil makes the application of the graphite much easier. A cracked valve seat, due to expansion strains or to the hammering of the valve, is a common cause of compression leakage, and is rather difficult to locate as the leakage only occurs under comparatively high pressure. Leakage may also occur between the valve cage and the cylinder casting unless pains are taken to thoroughly clean the cage and the bore before fastening into place. Warped valves are caused by overheating, the head of pallet of the valve becoming out of square with the stem, or by twisting on the valve seat. If warped valves are suspected the high point of the seat may be determined by means of the following test and should be carefully filed down until it is close to a bearing after which it may be ground down as described under pitted valves. If the stems are now in good condition examine the seating surfaces of the valve pallets and cage or rings. The seats should be bright and free from pits, depressions, or streaky blue discolorations. If the seats are deeply grooved from long continued leaks it is best to discard them and replace with new. Pitted valves, and those slightly grooved or streaked should be reground by the use of a little emery flour and tripoli which operation is performed as follows: Lift the valve from its seat and apply lubricating oil to the seating surface, then sprinkle a little flour or emery on the oiled surface and drop the valve back on the seat. Do not use coarse emery nor too much of the abrasive, a pinch is enough and will grind as rapidly as a pound. Take care to drop the emery only where required, do not sprinkle it over the engine or working parts as it will cause cutting and the destruction of the bearings. Now turn the valve around in one direction for about a half dozen turns and then in the other direction for the same length of time, alternately, at the same time applying a moderate pressure on the valve. Small valves may be rotated with a large screw driver entered in the slot found on the valve plate, but the handiest method is with a carpenter’s brace in which is inserted a screw-driver bit. Never turn the valve around and around in one direction continuously as this movement is liable to cause grooving, alternate the direction of rotation frequently with occasional back and forth movements made in a semi-circle. Do not press heavily on the valve, use only enough pressure to insure contact between the two seating surfaces.