When you find that it is necessary to inspect the cylinders or pistons of an air compressor, you should refer to the manufacturers instruction manual and the appropriate Planned Maintenance System (PMS) maintenance requirement card (MRC) for the proper methods and tools required for removing the heads. The following is general information on removing a piston, fitting new piston rings, and checking piston end clearance for a vertical compressor.

To remove trunk pistons from vertical compressors, it is first necessary to remove the cylinder heads. In the case of three- or four-stage compressors, the third and fourth stage cylinders will also have to be removed. Next, you must turn the compressor by hand to top center and remove the lower half of the crank pin bearing, or the entire crank pin bearing box, if so directed. Pull up the piston and connecting rod through the cylinder. In removing pistons from compressors fitted with cross heads and piston rods, the general procedure is to loosen the piston rod locknuts adjacent to the cross head, then unscrew the piston rod from the cross head and lift the piston and the rod out of the cylinder.

If the piston rings are worn or broken and require replacement, take accurate measurements of the cylinder or liner to determine the exact diameter. Standard size rings may be used in over-size cylinders if the oversize does not exceed 0.003 inch per inch of the cylinder diameter. If the cylinder or liner is worn beyond the manufac-turers recommended limits, it should be replaced. On compressors not equipped with liners, such as air-cooled or large pneumatic service compressors, one reboring is allowed.

When piston rings are being replaced, they should first be fitted to the cylinder to check for proper end clearance. If necessary, the ends should be filed until the manufacturers recommended clearance is obtained. Rings should then be fitted to the piston grooves, making sure that the side clearance of each ring is such as to allow it to fall into the groove by its own weight. The thickness of each ring should be checked to make sure that the groove is deep enough for the ring thickness. In replacing the rings, care must be taken to ensure that they are staggered so that the ring splits are not in line. After the piston is assembled, it is advisable to wire the rings tightly with a soft copper wire so that they will enter the bore without difficulty. As each ring enters the bore, the binding wire can be removed through the valve ports. Most cylinder liners are beveled at the top to permit the rings to compress and enter the bore easily. In certain designs of differential pistons, it is necessary to loop a wire around the top of the lower liner, using valve ports for access, and squeeze each ring closed as it approaches the top of the liner. One technique is to slip the rings over a thin piece of shim stock which, in turn, is slipped over the piston. This prevents overstretching and other possible ring damage.

After a piston has been replaced in a compressor and the head has been drawn down evenly, it will be necessary for you to check piston end clearance. NOTE: The piston end clearance must always be measured and adjusted after replacing the pistons, or after any adjustment or replacement of the main, crank pin, wrist pin, or cross head bearing has been made. This is accomplished by inserting a lead wire through a valve port or indicator connection, and then jack-ing the compressor over so that when the piston has moved to the end of its stroke (top dead center (TDC)), the lead will be flattened to the exact amount of clearance at the end of the stroke. The lead wire should be long enough so that the reading can be taken at a point near the center of the piston.

In differential piston compressors in which compression takes place in two stages during the same stroke, the clearance must be measured for both stages. In double-acting compressors, or in those compressors in which compression in any stage takes place on the reverse stroke, a second reading of the clearance at the bottom of the stroke (bottom dead center (BDC)) of that cylinder must be taken.

After taking these readings, you can adjust the piston end clearances. Proper clearances for a specific compressor are usually found in the manufacturers instruction book or on the blueprints. If neither of these documents is available, then you should adjust the clearance so that the TDC stroke is approximately 1/64 inch greater than the BDC stroke. This is necessary to allow for the expansion of the running parts when the compressor is in operation.

The method of adjusting the piston clearance varies with different compressors or designs. Adjustments may be accomplished by one of the following methods: (1) by adding or removing shims in the connecting rod between the lower end of the rod and the crank pin bearing boxes; (2) by adding or removing shims between piston and socket type wrist pin boxes; (3) by screwing in or backing out the wrist pin adapter, in compressors having a threaded wrist pin adapter; (4) by turn-ing the piston rod in or out of the cross head, in double-acting pistons equipped with piston rod and cross head. In some compressors, the piston clearance is nonadjustable.