Piston rings seal the gap between pistons and engine-block cylinder walls to regulate combustion gases and the flow of oil. Usually there are three rings per piston, two compresson rings and an oil ring. Setting the proper gap on piston rings is important because they endure tremendous heat and pressure. Without exact fits, rings can not perform their job. Piston rings have end gaps that must be set when they are installed. These measurements must be exact to achieve peak combustion and performance.
Make certain all cylinder bores have been cleaned out or honed before fitting any rings inside for measurement. Use an oil spray-lubricant to spray each cylinder bore from top to bottom, leaving a thin coat of oil inside each cylinder.
Refer to your repair manual's specification guide for the correct ring end-gap for your engine. You will need the make, model and year of your vehicle, along with its engine size in cubic inches, bore diameter and piston stroke. As a general rule, most engine manufacturers advise a minimum end gap for the topmost compression ring of .004 inches multiplied by bore diameter. For example, a 4-inch bore would translate to .016 inches (4 inches x .004 inches = .016 inches). The second compression ring has a standard formula of .005 times the bore diameter. A 4-inch bore would translate to .020 inches for the ring end gap. The standard for all oil rings, regardless of engine size, is .015 inches. Always follow the measurements listed in your repair manual.
Assign each set of three rings to a specific cylinder bore and write it on a piece of paper. Keep the rings in order. Place a top compression ring in its respective cylinder, about an inch or so below the top. Take the measurement of the ring by fitting the right-sized feeler gauge into the gap between the ring. If it falls within specifications, note that on your piece of paper. Push the same ring down to the bottom of the cylinder, levelling it in the bore, and measure that ring gap with the feeler gauge. Record that measurement. Do the same with the second compression in the same bore and record the gap measurement. Finish measuring the oil ring in the same fashion and record its measurement.
Examine the numbers on your piece of paper and determine if the bottom numbers show a significantly smaller end gap. If so, it means the cylinder bore has a more tapered wear at the top. Use the bottom number to set the correct end gap for that cylinder. Use a fine-tooth file to gently file the insides of the ring end to remove enough metal material to meet specifications. Support the ring on a wooden block while filing. If you end up with ring end-gaps that are larger than the ring gap you measured with the feeler gauge, get new rings.
Refer to the repair manual's maximum taper wear for your engine's cylinders. If the cylinders are beyond specifications, you will have to purchase new rings.