The Secrets To File-Fitting Piston Rings Successfully

Piston rings perform a number of duties in an engine. They seal combustion pressure and gases in the chamber, preventing them from passing by the piston and into the crankcase where they can only do harm. They stabilize the piston as it travels up and down the cylinder bore and they help cool the piston by transferring most of the piston’s heat into the block’s cylinder bore, and finally, they meter the film of oil on the bore surface during operation. Rings perform these functions under the most extreme conditions, which is why premium ring sets should always be used, with materials and designs matched to the intended application. Paying attention to the small details, like properly file-fitting the piston rings to adjust the end gap, is critical to the performance of the engine.

Piston rings come in sets for each piston. Ring type and material depend on the application and desired usage. The typical ring package consists of two compression rings and an oil ring.

Piston rings come in sets for each piston. Ring type and material depend on the application and desired usage. The typical ring package consists of two compression rings and an oil ring.The vast majority of piston and ring designs use a 3-ring package. The top two rings are called compression rings, and seal the combustion pressure. The bottom oil ring is actually made up of three different rings—two narrow rails and one expander. The oil ring’s main function is to control the amount of oil that makes it between the piston and bore, which is necessary for lubrication and heat-transfer purposes, but the second ring also does a bit of oil control. Too much oil past the rings ends up in the combustion chamber, which can lead to all kinds of problems; the oil control ring’s function is critical.

It’s a good idea to wipe down the rings as they’re being sorted. These tips and tricks are coming from CNC Motorsports in Brookings, South Dakota, using Total Seal piston rings on Wiseco pistons.

Piston compression rings seal to the bore via radial tension and combustion pressure, and each needs to be taken into consideration when fitting a set of rings to an engine. Radial tension is what you feel when you compress the ring by hand–that push-back as the ring wants to reopen– and it helps push the ring into the bore surface and seal the gap between piston and cylinder wall. In operation however, much of the sealing comes from combustion gases that slip behind the ring (between the ring and piston groove), and that combustion pressure helps push the ring away from the piston and into the bore.

Piston rings expand due to the heat of combustion which closes the end gap. Too little clearance and the ring ends will butt, causing damage.

The most critical thing to check when installing piston rings is the end-gap. Ring end-gaps must be properly set to make sure the gap allows minimum combustion pressure to pass between piston and bore. A ring must have a split in it so you can slip it over the piston top and into place. This gap should be as small as possible during operation to seal as much combustion pressure into the chamber as possible, but large enough to keep the ring ends from touching when they expand due to combustion heat. This is called butting and will cause catastrophic piston failure.

Ring side clearance is the space between the piston’s ring land and the ring itself. You don’t have to install the ring on the piston to check side clearance, you can just slip the ring in the groove and measure the clearance with a feeler gauge. While the ring is positioned in the groove, check to make sure the ring width itself isn’t deeper than the groove. This is known as backspacing.

On many performance applications, companies like Wiseco Pistons include file-fit rings–often manufactured by Total Seal, as these are–which are designed with very small end gaps that must be filed to fit in the bore in order to end up with the proper gap for the application. To check the gap, the ring should be placed into the bore (without the piston) and squared up so that it’s level, using a ring-squaring tool. The cylinder bore must be honed to its final size before ring gap can be checked. Once the ring is in the bore, use a feeler gauge to check the gap.

Use a ring squaring tool–offered by ring manufacturers like Total Seal–to seat the piston ring squarely in the bore, and a feeler gauge to check its gap. Check the gap often and grind slowly to prevent ruining the ring.

To adjust the gap, the ring must be filed. You can use a flat file secured in a vise or there are electric ring filers the job easier and more precise. Make sure to file from the outside of the ring to the inside as this prevents chipping of the ring face and potential coatings. Make sure the cuts are straight by compressing the ring and holding it up to the light to look at the gap. If it’s not perfectly straight, work the gap a bit with the file making sure you’re not opening it up more than the final gap required.

Use a feeler gauge to determine the preliminary ring gap. This process needs to be repeated for every ring, including the oil ring.

When file-fitting the rings, sneak up on the final gap. Go slowly, removing minimal material at a time and re-checking the gap often. Remember, you can’t put material back.

The gap is adjusted by filing the ends of the ring. The cheapest method is clamping a file in a vise and running the ring across the face. There are two types of dedicated ring filers: hand-drive and motorized. This shop uses hand filers but modifies them to spin with a drill, when desired. The motorized tool offered by Total Seal accomplishes the same goal. Different strokes for different folks.

In ideal operation, the ring end gaps will be near zero to trap all of the combustion pressure in the chamber. But, don’t be tempted to run ring end gaps smaller than the recommended spec (included with all ring sets). The potential for ring butting and damage far outweighs any potential power gains.

Grind slowly, and make sure the gap is perpendicular to the bore and not angled.

As for installation order, the top and second rings look very similar to each at first glance, but their designs are often quite different. The top ring is subject to the most intense heat and pressure, and the second ring is its “backup” and does double duty metering oil that makes it past the lower, oil ring.

Check the instructions from the piston manufacturer to determine the proper ring gap for the application. Power adder engines will require larger end gaps than normally aspirated engines.

Because of this, their designs are sometimes different, so you need to make sure the right ring goes in the right piston groove. Ring sets identify the top and second rings, usually by markings on the top of the ring surface.

Once the file-fitting process is complete and the rings are all gapped correctly, the rings can be assembled onto the pistons and installed into the engine.

Photos: Mike Magda

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