When fitting new piston rings, the end gap is a crucial measurement. In order that a ring may be fitted into the "grooves" of the piston, it is not continuous but is broken at one point on its circumference. The ring gap may be checked by putting the ring into the bore/liner (squared to bore) and measuring with a feeler gauge. End gap should be within recommended limits for size of bore and intended "load" of compressor. Metals expand with a rise in temperature, so too small a gap may result in overlapping or bending when used under hot running conditions (racing, heavy loads, towing), and even at normal temperatures, a small ring gap may lead to ring gap closure, ring breakage, bore damage and possible seizure of the piston. Too large a gap may give unacceptable compression and levels of blow-by gasses or oil consumption. When being measured in a used bore it may indicate excessive bore wear or ring wear.(Radial wear on ring face reduces thickness of used/worn ring (face wear in bore) essentially decreasing face circumference of ring and thereby increasing size of ring end gap.)

Replacement of the piston rings should be made when they have worn to one-half of their original thickness. When piston rings have worn to this extent, excessive blow-by and reduced cylinder efficiency will occur if they are not replaced.

When ordering new piston rings, the cylinder bore diameter should be measured with a suitable inside micrometer so that it can be determined whether “standard” or “oversize” piston rings are required. Measure the bore diameter at several points to check for out-of-roundness. Supply these readings to the supplier if possible when ordering new piston rings.