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Before putting pistons back in casing, be sure there is no lint left in the ports or later this will come out and foul the action. Also, before putting the pistons back in casings, place a few drops of oil on the pistons. Use a light, high grade valve oil. Many players get good results by putting a few drops of oil on the piston, then running cold water over it and working it into the casing. Some players prefer to use water or saliva on new, tight-fitting valves, but generally speaking a fine oil is best. On valves which are worn, a heavier oil can be used, as this heavier oil helps seal up the excess clearance which has worn between the walls of the piston and casing.

Piston valves are very accurately fitted with clearance from .0012" to .0018" (one and two-tenths of a thousand to one and eight-tenths of a thousandth of an inch). The clearance on a side is just half this, or about .0008" (8 ten-thousandths of an inch) or the diameter of an average human hair. This does not leave much space for the film of oil, but a fine grade of oil will work all right. Valves that wear until the clearance reaches .0025" to .003" are liable to leak. Only a heavy oil which helps close up the excess clearance will make them playable.