How to fit piston rings
Correctly fitted rings give good compression and low oil use. Follow these steps, and always use the figures in the workshop manual for your bike.
Know your rings
- Top ring - the main compression ring, nearest the crown.
- Second ring - helps seal compression and scrapes oil down the bore.
- Oil control ring - on most four-stroke engines a three-piece ring: two thin rails with an expander (spacer) between them.
Two-stroke engines normally have one or two compression rings and no oil ring. The rings sit over locating pegs in the grooves, so the gaps must line up with the pegs.
Fitting step by step
Work clean, take your time and never force a ring - an over-stretched ring will not seal.
Check the end gap
Push each ring squarely into the bore with the piston crown, then measure the gap with a feeler gauge. Compare it with the figure in your manual.
Fit the oil ring first
Fit the expander into the bottom groove, then the lower rail, then the upper rail. Make sure the expander ends butt together and do not overlap.
Fit the second ring
Open the ring only as far as needed to slide it over the piston. The marked side faces up, towards the crown.
Fit the top ring
Fit it the same way, marked side up. On many engines the top and second rings look alike - check the markings before fitting.
Space the gaps
Turn the rings so the gaps are spread around the piston, as shown in your manual. On two-strokes, the gaps must sit on the locating pegs.
Oil and fit
Lightly oil the rings and bore with clean engine oil, compress the rings and slide the piston in without twisting.
Bed the new rings in
For the first few hundred miles vary the engine speed and load, and avoid long runs at one constant speed or labouring the engine. Change the oil after the running-in period.
Common mistakes to avoid
Rings upside down
The marked face must point up towards the crown
Expander ends overlapped
The ends must butt together or the oil ring will not seal
Gaps lined up
Lined-up gaps let compression and oil pass
Glazed bore
New rings need a freshly honed bore to bed in
Parts for the job
Ring questions
How do I tell the top ring from the second ring?
The top ring is often chrome-faced or barrel-faced, and the second ring often has a tapered face and a darker finish. Both usually carry a mark that faces up. If in doubt, check your workshop manual or contact us.
Can I re-use my old rings?
We do not recommend it. Old rings have worn to the old bore and rarely seal well again. New rings are a small cost compared with stripping the engine twice.
Do I need to hone the bore?
Yes. A light hone (deglaze) gives the bore the cross-hatch finish new rings need to bed in. A rebored cylinder is honed as part of the job.
What if the end gap is too small?
Do not fit the ring - a gap that closes when hot can break the ring and damage the bore. Check that you have the correct size for your bore, then contact us.
Need the right ring set?
Tell us your bike and bore size and we will find the correct rings for you.