Turning a New Piston.—Viewed from all aspects, it is advisable to purchase the new piston from the engine builder. Frequently, due to high repair prices or delay in delivery, it becomes ad vantageous to have the piston cast at a local foundry. The old piston can be used for the pattern. A core box can be cheaply made since the piston bosses and ribs can be cut in the core itself. The success of the entire process depends on getting the Core central in the mold, having the risers of considerable height, and in using iron that has but a small amount of scrap, using no stove or agricultural scrap at all.
In many instances the cylinder is not scored nor worn 'out of round, and so it does not demand reboring. If the piston be purchased oversize from the manufacturer or be cast at the local foundry, it is necessary for the engineer to have the piston turned to the correct size. It is usually best to have this job completed at a machine shop. In those installations where the .engineer is expected to do his own machine work, this undertaking need not be dreaded. The main thing is to exercise great care in centering it in the lathe. The best method is to first place the piston on the lathe, chucking the head. A light cut should be taken off the inside of the bottom end, as well as on the edge, to square up this end of the piston, using the steady rest to support the weight of the piston. The next process is to reverse the piston, chucking from the inside surface that has just been finished. Still using the steady rest, the head of the piston is finished up. A drill is inserted in the tail stock, and the center is counter-bored to receive the center of the tail stock. The steady rest is then re moved, and the piston tested for trueness. If it is a little out, this should be corrected. A roughing cut is next taken over the body of the piston, ending with a finishing cut. If the piston pin bosses must be bored, the best procedure, on an ordinary lathe, is to block the piston up on the carriage and bolt the boring tool to the lathe face-plate. A very light cut should be fist taken, to check the lining up of the piston. If it is not blocked up true, this reveals the error. The ring grooves can be made while the piston is chucked on the lathe.
After taking the finishing cut on the piston, a file should be used to give the surface a good finish; the use of emery cloth to follow up the work of the file is also a good plan. It is the
practice of most builders to give a slight taper to the piston from the first ring to the edge of the head. In turning a new pistbn, the engineer should taper it at this point, making the variation in diameter about inch.
Distorted encounters a great many instances where the cylinder is badly scored and where the operator can not account for this damage, he being insistent that the piston has been supplied with plenty of lubrication. Usually investi gation reveals that the piston walls are not strengthened by supporting ribs, and the piston pin is locked at both bosses by set-screws or similar devices. If the piston pin becomes heated, it will tend to lengthen. When there are no ribs to resist• this force and the pin is held at both ends, the thin piston walls assume an elliptical shape. This always results in the cylinder cutting along the sides in line with the pin bosses. If the engineer finds that the piston is built on similar lines, it is well to file .a flat surface on the piston around the bosses. Then, in case of elongation, this clearance will allow the piston to change its shape without damage to the cylinder.
Where the damage already exists, it is not always necessary to rebore the cylinder, even though the scoring be deep. If the scoring or cutting is limited in area and does not extend along the entire stroke, the damage can be remedied by using an emery stone, file and scraper. In such instances good results are ob tained by first using an emery wheel held in the hand. By rubbing the scored spots in this way the rough ridges are re moved, and a file and emery stone, finishing up with the scraper, will smooth the work. This process has been used on many occasions where the cylinder looked hopeless.
Piston Rings.—In cases of worn piston rings, the best plan is to replace them since their continued use means loss of power. There is a prevailing habit of engineers to let the rings stay in, no matter how badly worn, if the engine con tinues to pull its load. Usually the engine is underloaded, and its ability to take care of the plant's demand is no criterion as to the suitability of the piston rings. In most plants the best way to decide the problem of ring renewal is to depend on the fuel consumption. If this shows an increase, and the rings are worn, then it is time to replace them.