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Piston ring gap
Too small a ring gap closes up when the ring gets hot and can seize. Enter the bore and choose the application, and you get recommended gaps for all three rings, based on CP-Carrillo's guide values.
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Top ring
0,43 mm
The gaps are filed by hand with a ring filer and apply to rings meant for the bore. Gap = factor times bore.
How the calculation works
The ring expands with heat, and the expansion grows with the ring's circumference, that is with the bore. So the gap is given as a factor per unit of bore. A hotter engine with high boost or nitrous needs a larger cold gap so the ring does not close up and lock when it gets hot.
The top ring gets the smallest gap of the compression rings, since it handles most of the sealing. The second ring gets a larger gap, so that gas leaking past the top ring can move downwards instead of building up between the rings and lifting the top ring so it loses its seal. The oil ring has a fixed factor. The measurements apply to rings meant for the bore, and the gaps are filed by hand with a ring filer, straight and free of burrs.
Example
A bore of 96 mm for street use gives a top ring gap of about 0,43 mm, second ring 0,52 mm and oil ring 0,36 mm.
Gap factors by application (gap = factor × bore)
| Application | Top ring | Second ring |
|---|---|---|
| Street, naturally aspirated | 0,00446 | 0,00546 |
| Hi-perf street, MC, ATV | 0,00496 | 0,00596 |
| Drag race, naturally aspirated | 0,00546 | 0,00645 |
| Circuit racing | 0,00596 | 0,00695 |
| Forced, low pressure | 0,00546 | 0,00645 |
| Forced, medium pressure | 0,00620 | 0,00719 |
| Forced, high pressure | 0,00695 | 0,00893 |
The oil ring factor is 0,00372 regardless of application.
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Common questions about piston ring gap
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