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How to determine piston ring size

Updated July 19, 2017

Without piston rings to keep the combustion chamber sealed, a motor would just be a low mileage smoke maker. As one of the most underrated components of the combustion engine, piston rings do more than align the piston within the cylinder walls. Top, middle and bottom piston rings must seal the burning of fuel on top of the cylinder, lubricate the cylinder walls, and clean up extra oil left behind. Because they are slowly becoming smaller and thinner than ever, how to determine piston ring size depends on who you talk to and how you plan to drive your car. Once you determine which composite material is right for you, fitting the ring inside the cylinder takes skilful sanding and a little patience.

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Measure the inside diameter of the cylinder bore with an inside diameter indicator. Take a number of readings to discover the true inside diameter of the cylinder bore. After inserting the gauge, square it in the bore so that you reach the correct reading. Move the gauge up and down inside the cylinder until you are sure that you have an accurate reading. Use a micrometer to measure the diameter from the top of the cylinder and compare the readings to confirm that they are accurate.

Determine the width of the piston from edge to edge. The difference from the readings of both cylinder wall inside diameter and a piston's outside diameter will assist you in getting the right set of piston rings for your engine. Keep track of the measurements after they have been made. Use a notebook to save the measurements in writing for future use.

Measure the gaps that are the piston ring grooves. Both width and depth will need to be discovered so that when you install a new set of piston rings, they will fit correctly. Measuring the gaps with old, used rings can give you the information you need to correct gaps that have increased. If new rings can be purchased to overcome the wear, have the rings seated and gapped properly.

Gauge the gaps of piston rings with a new or old ring in place using a feeler gauge to measure the distance in between the ring gap and the ring when seated against the other gap wall. Determine the size of the space behind the ring, between the ring groove base and the back of the piston ring.

Contact a piston ring dealer or manufacturer like Otto Gas Engine Works ( Order a set of new piston rings using your piston and cylinder wall measurements. Tell the piston ring dealer about future driving conditions so they can help you decide which alloy works best for you. Beyond getting the right size piston ring, it is equally important to select the right material for best results.


Use gap spacers instead of increasing ring width to avoid added friction. For the family car it is better to use the original sized piston rings instead of changing the size of the rings to a larger thickness. Instead of machine honing, try hand sanding with an emery board to eliminate imperfections inside the cylinder bores.


High performance piston rings will perform better but wear faster than standard configuration rings like the ones on most family cars and trucks. As you move to thinner, more exotic piston rings, decide if the extra 30 horsepower is worth changing piston rings every year.

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Things You'll Need

  • Feeler gauges (.001-.010)
  • Micrometer
  • Inside diameter indicator
  • Piston
  • Piston ring square
  • Cylinder bore
  • Notebook

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.

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