Wrench Tip – Gapping Piston Rings

Here is the ring all squared in the bore and ready to measure.  Note the really smooth bore finish and the lustrous WPC treated surface.

Howard Watanabe uses a feeler gauge to measure the ring end gap.  A rough rule of thumb is that you want 0.004 x inches of bore size to determine your top ring gap.  You multiply your bore size by 0.0025 per inch of bore for the second ring gap since it sees less heat and expands less.  For this engine with a 4.166 bore that would be a gap of around 0.017″ for the top ring and 0.010″ for the second ring.  If this was a street engine with cast pistons we would probably stick with something like that but for a racing engine, particularly a turbo one we will open up the gap a little to reduce the chances of the ring ends butting up against each other when expanded by heat.  A rough rule of thumb is to add 0.002″ to the gap of an extreme use engine. We will use a gap of 0.019 for the first ring and 0.012″ for the second ring to allow for additional expansion.  If in doubt contact your piston manufacturer for advice.

Howard uses this hand powered ring gapping tool to grind the gaps to specification.  He removes a very small amount of material and rechecks each ring several times.  Remember you can always grind more but you can't put material back!  You could use a small jewelers file if you don't have this device but why bother, it is cheap and you can get them almost anywhere, such as Summit racing.

After the proper ring end gap is obtained, Howard uses a knife sharpening stone to break the cut edges of the end of the ring.  This prevents cylinder wall scratching while the engine is running.  Note that only the freshly filed end of the ring is deburred, not the whole ring which would ruin it!