How to File-Fit and Install Piston Rings

November 27, 2018 / by Mike Magda

Perhaps the most critical engine assembly step is file fitting and installing the piston rings with the correct gap. We walk you through the process step-by-step in this story. 

The piston rings are the key defense against cylinder pressure leaking into the crankcase, thereby leaving horsepower on the table. Rings also control oil and transfer heat. They’re valuable tools in the performance of any engine, and that’s why engine builders pay close attention to this tedious but very essential assembly step.

Correctly sizing of the gap for each ring is important because of heat. As the engine warms up, heat from the combustion chamber will force the rings to expand. A gap that is too tight may result in the rings binding (butting) within the groove and possibly breaking. A gap that is too wide will allow combustion pressure to fill the crankcase and reduce horsepower. Different engine setups and different fuels result in different thermal expansion properties within the piston, rings and cylinder walls—so the recommended ring gap changes between applications. Manufacturers take all these factors into consideration when developing gap recommendations for the engine builder.


Installing piston rings correctly will allow your engine to maximize both power and longevity. Read on for a step-by-step walkthrough.

Another reason for the ring gap is simply because the ring must be split apart slightly in order to facilitate installation. There are dedicated tools, such as ring-expanding pliers that help engine builders install rings by holding the ring and expanding the gap just enough to clear the ring over the top of the piston and position the ring into its respective groove.

Some mechanics have developed hands-only installation techniques whereby one ring end is started in the groove, then the ring is spiraled around the piston and into its groove.

While the spiraling method is viable, modern, thin rings are far more susceptible to bending and require extra care to prevent that. Ring expanding pliers work very well but take care not to over-expand the rings. It’s also crucial that the rings be installed right side up, especially rings with an inside bevel. Most top compression rings have a dot on the surface that indicates the side that should face upward when installed. A ring that is installed upside down will lead to excessive oil consumption.

Finally, after all the rings are installed, rotate them around to ensure that they move freely within the grooves. Then orient the ring gaps according to the instructions to improve ring seal. This usually means the gaps for the first and second rings are opposite each other and perpendicular to the pin bore. The gaps for the three oil rings spread apart evenly around the piston. 

Following is a step-by-step installation guide, courtesy of CNC Motorsports in Brookings, South Dakota. Engine builder John Himley was installing Diamond pistons into a turbocharged small-block Chevy that will run on methanol, and so the gap was set accordingly.

The pistons were also designed with a short compression height, which forced the wrist-pin bore into the oil ring groove. A support rail was then necessary to provide a solid surface that the oil ring can rest without fear of bending into pin bore. As you’ll see in the photos, this support rail must be installed correctly for optimum performance.

After removing the rings from the packaging, clean them with a cloth and solvent. Next, separate and organize the rings—compression, second and oil rings––for each cylinder. Some engine builders simply lay them out on the table while others have dedicated placeholders.
Position a ring in its respective groove to check the radial back clearance by making sure no part of the inside diameter of the ring protrudes past the ring land. Use a feeler gauge to measure the exact clearance.
Review the manufacturer’s recommendations for the ring gap. As you can see by this chart, boosted nitro engines require much more gap than street engines.
To determine the initial gap, position the ring inside the cylinder near the top of the bore. For some rings, the ends may overlap for this initial test, and you’ll have to grind away just enough to get them positioned inside the bore.
The ring must be square inside the bore relative to the top of the deck before checking the gap. Some engine builders use an old flat-top piston of the correct bore to push the ring about an inch inside the cylinder. There are also dedicated tools that will correctly square up the ring.
Use a quality feeler gauge to determine the gap; again checking that measurement against the manufacturer’s recommendations. 
A dedicated ring-filing tool—whether manual or power operated—should be used to file the ends of the rings to the correct gap. This is a manual tool that has been converted to power with an air drill.
Here’s an example of a manual filing tool. Be sure to “sneak up” on the correct gap. Don’t remove too much material at once, and also try to remove the same amount of material from each side.
Lightly file off any burrs or rough edges before installing the ring in the bore for checking.
Double check that the ring gap is correct and that the edges of the rings are parallel to each other.
Once the pistons are secured to their respective connecting rods, the rings can be installed.
On many racing pistons, the wrist-pin bore invades the oil ring groove, so a support rail must be installed first. The support rail has a dimple that must be identified and installed in the pin bore to keep the rail from rotating.
Now the oil ring expander—also called the oil separator ring—is installed followed by the upper and lower wiper rings. These rings don’t need an installation tool and spiral around the piston crown rather easily.
The middle ring is installed. The key is to start the ring with one edge and gently spiral the ring around the piston into its respective groove.
Now the top ring is installed. Make sure the gaps of all of the rings are spaced 90 degrees apart from eachother.
Be sure to use a quality ring compressor to prevent damage to the rings while installing the piston into the cylinder bore.

Topics: ENGINE BUILDS, featured, PISTONS 101, ENGINE TECH, Tech

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Written by Mike Magda

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