Frequently, these new rings must be made at a local shop. In such cases it is good practice to have the rings rough-turned to an outside diameter about inch larger than the cylinder. The ring should then be cut and, after being clamped to the desired diameter, finished inside and out. This method insures a ring that will hug the cylinder walls at all times. Moreover, the ring should be made almost as wide as the groove; the difference should not exceed .01 inch. The ring should equal in thick ness the depth of the groove less .02 inch. If made thinner, the clearance will simply fill up with coked oil.
In removing piston rings, the best method is to start on the one nearest the piston end and remove each one successively. The easiest way is to use a file to spring of the ring so that an iron strip, such as a barrel hoop, can be inserted between ring and piston. The strip is worked around the piston, others being added as the process continues. The ring is now raised above the piston surface by the four strips, similar to Fig. 263. It is now easy to slip it off. The strips also prevent the ring from dropping into a groove as it is slipped off. In replacing rings, the same method can be followed.
Distorted Exhaust Bridges.—Another common cause of a worn piston is the distortion of the exhaust port bridges. The cylinder should be inspected at least every sixty days, and if the port bridges seem bright the piston will probably be found to be cut to some extent. ' The remedy is to file and scrape these bright spots until the piston clears them. At these inspections the piston should be pulled and the rings examined. If badly gummed, kerosene will usually loosen them. If this is not successful, placing the piston in strong lye will free the rings.
Fractured Piston Heads.
—The expansion of the piston head often causes minute fire cracks to appear; this occurs but seldom in conical head pistons. Frequently, one of these fire cracks will develop into a well-defined fracture several inches in length. Usually this extends entirely through the head and allows the gases to blow through, resulting in poor compression and decreased power. The tendency of the inexperienced engineer is to replace the piston at a heavy cost. This replacement is entirely unnecessary in the majority of cases. If the piston is 12 inches or over in diameter, the method described in Chapter VI is the most acceptable. In small engines this entails too much time and skill. A cheaper repair is shown in Fig. 264. Here a steel plate inch thick is fastened over the crack by means of a series of machine screws. Iron cement, such as smooth-on, should be coated over the piston face before the plate is drawn into place. This patch will hold the compression and will
strengthen the head. To prevent the crack from developing in length, a ;finch hole should be drilled in the head at the ends of the crack. The machine screw heads should be sawed off short so that they do not project into the cylinder space any great amount; if they do, they might cause preignition.
Piston Pins.—The usual type of piston pin consists of a straight cylindrical piece of steel, hardened and ground. In most designs it is held in both bosses by means of two set-screws, the pin being countersunk to receive the conical ends of the set-screws. In some engines the pin is prevented from turning by the use of a key. The use of a set-screw in each boss constrains the pin from having any opportunity to expand lengthwise without distorting the piston. As a safeguard, the engineer need have no hesitancy about removing one of these set-screws. He can then be a little more at ease as to the danger of the piston getting out of round. Due to the frequent preignition in the cylinder, the piston pin is subjected to extreme hammer blows. If it is not properly heat treated, a flat place will develop where it comes in contact with the bronze bearing. Where the wear becomes considerable, the pin should be rotated either a quarter or half turn, thereby pre senting a true surface to the bearing. When the pin has been so rotated, and flat places have been worn on all four sides, a new pin is necessary. On engines above 30 or 40 h.p. it is inadvisable to turn the pin since turning it results in con siderable play in the brasses. Where the pins are not a snug fit in the bosses, the latter tends to pound out of shape. In renewing pins, the bosses should be examined for wear. If the bosses are out of round, the cavities must be rebored, and the new pin must be made large enough to fit the new bore. Since the increase in diameter will be slight, the pin need not be reduced in diameter at the brasses. These can be enlarged to accommodate the new pin. Piston pins should, if at all possible, be procured from the builder. He is in a position to furnish a pin heat-treated and ground. When this is impossible, a pin can be made up from cold-rolled shafting. If facilities are at hand, it should be case-hardened and ground. Where the engineer is satisfied with a less expensive method, a lathe-turned pin, smoothed up with emery and with the two ends ground into bearing at the bosses, will suffice for a long time, though its wear will be much more than that of a hardened pin.