It's a well-known fact that piston ring gaps represent an area of pressure loss in a running engine. It has been found that minimizing end gap under operating conditions offers a measurable power increase. However, under that normal operating temperature, rings expand due to heat retention, and if there isn't enough gap, the gap may close entirely while the engine is operating—this is a bad thing. This condition is referred to as "butting" the end gaps, and the result can be disastrous. Traditionally, pre-gapped replacement ring-sets are "sized" for the nominal bore size of the engine, usually with generous end gap clearances to minimize the potential of butting the end gaps. In a serious performance application, custom sizing the end gaps to an exact specification will minimize the pressure loss through the gap, and enhance power. Manufacturers offer "file-fit" ring sets, normally spec'd at a .005-inch larger than the nominal bore dimension, providing an oversized ring, which can be custom fit to the cylinder bore.

While the potential advantage of file-fit rings is simple to conceptualize, the question becomes, what is the correct end gap? The required end gap can vary with the ring material, application, cylinder bore diameter, piston material, and top ring placement. Ring manufactures offer guidelines based upon their finding of what works in various applications. Some piston manufacturers have their specific specs on ring end-gap requirements when using their product, and these specifications should supersede the recommendation of the ring manufacturer.

How are the ring gaps actually brought to the correct size? Excess material is removed at one or both ends of the ring until the gap measures to spec. Trimming the ring gap can be done by a variety of methods. We know engine builders who hand cut the gaps with nothing but a quality draw file. Low-cost hand crank ring filing tools are popular with most casual builders, and more elaborate electric grinders with ring holding fixtures are also available, which speed the process. Basically, its cut and measure a little at a time until the desired dimension is reached.


The actual finished bore size of an engine's cylinder can vary by small fractions of an inch, depending upon such factors as the true piston measurement, and the intended clearance. Standard-sized rings cannot accommodate these small variations, and are generally produced to provide a generous ring gap. Custom file-fit rings, on the other hand, are precision fitted to the bore during the ring filing process.

Ring filing can be accomplished in several ways, from a simple hand file to hand crank filers, to even more elaborate power driven units such as the high-end unit shown here from Goodson. All will get the job done, but the professional machines will greatly reduce the time involved.

High performance file-fit rings will normally come with an instruction sheet with recommended end gaps for various applications and bore sizes. Some hypereutectic pistons, however, call for a larger end gap. Always give the piston manufacturer's recommendation priority.

Rings need to be test-fit into the bore, in order to measure the gap you're starting with. The ring needs to be squarely pushed into the bore to get an accurate reading. A ring squaring tool can be purchased for the task, or as shown here, an inverted piston with a second ring installed as a stop can do the job. Sometimes, out of the box, the ring's end gap may butt solid when trying to square the piston into the bore. In that situation, never force the ring in; file some material until the ring can be inserted without binding.

The end gap is measured with a feeler gauge. To prevent an erroneous reading from a ring that isn't fully seated, work up from a smaller feeler gauge that fits easily one thousandth at a time until the next one won't go in. Closely examine the end gap by feel with the feeler gauge as well as by sight to judge whether the end gap is parallel.

This motorized tool is first set up to square rings of the bore size to the abrasive wheel by adjusting the eccentric locator. The ring is only square at one distance from the abrasive wheel, so we set the slide .100-inch from the stop, and then adjusted the eccentric with the ring end just touching.

A manual ring grinder is a lower cost alternative, and requires repeated trial and error test fitting and measuring to sneak up on the desired fit. Some builders will file from only one side to manually align the cut squarely. Always cut with the wheel turning inward towards the center of the ring, as an outward cut will flake the moly from the ring face around the gap. My favored technique to control the parallelism is to press the ring firmly into the stop pins if more material needs removal from the outside of the gap. Alternatively, to take more from the inside of the gap, the ring is pressed lightly against the stop pins and squeezed more tightly in the center. You have to develop the feel.

After filing, the ring will likely have a burr at the cut edge. This will cause the ring to hang-up in the groove, and can also damage the piston. A small, fine, needle file can be used to lightly de-burr the edge, but don't chamfer it. The ring can be de-burred with a small machinist file, or as shown in the leading photo, the de-burring wheel of the ring grinding machine, if so equipped.

File fit rings will show their worth when lower leak down is noticed during a leak down test, due to the decreased flow area for gasses to escape. The added cylinder sealing means less blow-by into the crankcase, and more power at the flywheel.