How to File Piston Rings

Updated July 19, 2017

Piston technology is based on improved piston performance within engines that use internal combustion to produce horsepower and torque. Today's engines perform better than their outdated counterparts because of the improvements in design and materials within the cylinder walls of an engine made of metal. The goal of the piston ring is to decrease friction and metal-to-metal contact of the pistons within each cylinder. Piston rings also oil the cylinder and remove excess fluids that accumulate within each cylinder. New materials are what make modern piston rings different than their older, made of steel ancestors, but filing each ring to fit remains the same. Learn how to file piston rings correctly when your old engine needs new piston rings to eliminate oil and gas from passing into the exhaust and internal parts where no extra oil and gas should be.

Collect three piston rings per cylinder. New piston rings are constructed with extra material that needs to be filed away at the two ends of each ring. The gap that is left when the piston ring is installed allows for the proper combustion containment, oil flow and cylinder wall cleaning when completed properly.

Inspect the piston specifications of your vehicle. Find the correct gap of each piston ring before you begin to align and fit your new piston rings. With the information on piston ring gap you can begin to size and fit your new piston rings within the cylinder walls of your particular engine.

Insert one piston ring into the cylinder it will be used in. It is important to have the piston ring evenly inserted into the cylinder so that it is in the correct position for checking the gap distance. A piston ring alignment tool works the best. This tool is similar to an actual piston as it presses down onto the new piston ring and puts it in the correct alignment for measuring the gap distance from one side of the gap to the other.

Measure the gap of the new piston ring when it is positioned in the cylinder. Use a feeler gauge that can stack the feelers together to make a feeler gauge that is of the correct width. Take note of the width of the gap and then remove the piston ring for filing.

Grasp and secure a flat file into a table vice so that the file points toward you, and you can rub the inside of each piston ring gap to increase the gap that results from the piston ring being squeezed slightly within the cylinder walls. With the current measurement of the piston ring inside the cylinder wall, you can begin to know that by filing a certain amount off each side of the gap you will create a perfectly fitting piston ring when it is installed in the cylinder after filing.

Hold the piston ring with the leading edge of the rings pointed away. Before beginning to file the gap of the piston ring, you should understand that the ring is coated and with improper filing the coating can chip away, which results in lower performance and reliability. Always file small bits with the leading edge of the ring pointed away from you. Slide the piston ring gap on the file to slowly eliminate the extra metal that keeps the ring from properly fitting.

Check the gap size of the piston ring after filing by inserting the piston ring back into the cylinder and aligning it properly. Measure the gap using the feeler gauges and continue to file away little portions of the ring until the fit is perfect. Use a fine piece of sandpaper to remove any burring that may result from the filing of the piston ring gaps. Complete one ring and move to the next until all the piston rings are perfectly fit to your engine's cylinder walls to complete this piston rings project.


Use a piston ring measuring tool to check that your piston rings are properly sized before inserting into each cylinder for confirmation. Match each piston ring to one cylinder. Do not mix and match piston rings after filing.


Do not over file any of the piston rings. If the gap created is too large, you must replace the bad piston ring with another new one to be filed. Over filing of piston rings makes the piston rings unreliable when installed. Do not install piston rings that are not filed properly or be open for fatal internal damage to the pistons and cylinder walls from blow by of fuel and oil into the internal engine components and exhaust system, which results in heavy smoking out the tailpipe.

Things You'll Need

  • Combustion engine
  • Piston rings
  • Pistons
  • Sandpaper
  • Feeler gauge(s)
  • File
  • Vice
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About the Author

Francis Walsh has been working as a freelance writer since 2003. He has contributed to websites such as Shave, Autogeek and Torque & Chromeas, as well as provided content for private clients. Walsh has worked as a performance part-packer and classic car show promoter, now serving as crew chief for Nitrousfitz Racing.