Repairing Scored Cylinders
If the engine has been run at any time without adequate lubrication, one or more of the cylinders may be found to have vertical scratches running up and down the cylinder walls. The depth of these will vary according to the amount of time the cylinder was without lubrication, and if the grooves are very deep the only remedy is to purchase a new member. Of course, if sufficient stock is available in the cylinder walls, the cylinders may be rebored and new pistons which are oversize, i.e., larger than standard, may be fitted. Where the scratches are not deep they may be ground out with a high speed emery wheel or lapped out if that type of machine is not available. Wrist pins have been known to come loose, especially when these are retained by set screws that are not properly locked, and as wrist-pins are usually of hardened steel it will be evident that the sharp edge of that member can act as a cutting tool and make a pronounced groove in the cylinder. Cylinder grinding is a job that requires skilled mechanics, but may be accomplished on any lathe fitted with an internal grinding attachment. While automobile engine cylinders usually have sufficient wall thickness to stand reboring, those of airplane engines seldom have sufficient metal to permit of enlarging the bore very much by a boring tool. A few thousandths of an inch may be ground out without danger, however. An airplane engine cylinder with deep grooves must be scrapped as a general rule.
Where the grooves in the cylinder are not deep or where it has warped enough so the rings do not bear equally at all parts of the cylinder bore, it is possible to obtain a fairly accurate degree of finish by a lapping process in which an old piston is coated with a mixture of fine emery and oil and is reciprocated up and down in the cylinder as well as turned at the same time. This may be easily done by using a dummy connecting rod having only a wrist pin end boss, and of such size at the other end so that it can be held in the chuck of a drill press. The cylinder casting is firmly clamped on the drill press table by suitable clamping blocks, and a wooden block is placed in the combustion chamber to provide a stop for the piston at its lower extreme position. The back gears are put in and the drill chuck is revolved slowly. All the while that the piston is turning the drill chuck should be raised up and down by the hand feed lever, as the best results are obtained when the lapping member is given a combination of rotary and reciprocating motion.