Ordering rings or a piston? Measure these three things first

by Performance Saw Supply

The most common reason a piston or ring set comes back is not a bad part. It is a part that is one size off from the engine it went into. These three measurements catch almost all of it.

1. Ring thickness and top-ring shape

Engines with the same bore can use rings of different thickness, and some pistons use a tapered top ring while others use a flat one. If you are buying rings only, measure the thickness of your old top ring (or the width of the top groove) with calipers. Look at its profile too. A ring that is too thick will not go into the groove, and a flat ring will not seal in a groove made for a tapered ring. This matters most on Briggs & Stratton V-twins and Vanguards, Kawasaki FB, FC, FR, FS and FX engines, and Kohler Command twins.

2. Bore size

Measure the cylinder bore, or the old piston across the skirt at a right angle to the wrist pin. Saws and cut-off saws are sized in millimeters, and some models were built in more than one size. A Hilti DSH900, for example, can need a 54mm or a 56mm piston. Four-stroke engines are often rebuilt oversize, at .010, .020 or .030 over standard. Rings and pistons must match the bore you actually have.

3. Wrist pin size

If you are keeping your old connecting rod, measure the wrist pin or the small-end bore of the rod and compare it to the new piston's pin. This is especially important on Onan and Kohler twins.

Quick checklist

  • Model and spec or type number written down
  • Bore measured in two directions
  • Old top ring thickness and profile checked
  • Wrist pin diameter checked if you are reusing rods

Send us those numbers and we will confirm the right part before it ships.

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