Which piston size do I need?
Pistons are sold in a standard size (STD) and in oversizes for cylinders that have been rebored. Choosing the right one is simple once you know your bore size - this guide shows you how to check it.
STD or oversize?
STD (standard) pistons are made for the original factory bore. Choose STD if your cylinder has never been rebored and is still within the wear limit in your workshop manual.
Oversize pistons (+0.25mm, +0.50mm, +0.75mm, +1.00mm and so on) are for cylinders that have been rebored larger. The number tells you how much bigger than standard the piston is - for example a +0.50mm piston for a 66.00mm engine measures 66.50mm.
Each IMD product page shows the standard bore for that model and every size we stock.
How to check your bore size
You will need a bore gauge or an internal micrometer and the workshop manual for your bike.
Clean the cylinder
Remove the barrel and clean off carbon and oil so the gauge sits on bare metal.
Measure in three places
Measure near the top, the middle and the bottom of the ring travel.
Measure in two directions
At each height, measure front-to-back and side-to-side. Bores wear oval, so the readings can differ.
Use the largest reading
Write down the biggest figure and compare it with the standard bore and wear limit in your manual.
Check the old piston
Look at the piston crown - an oversize is usually stamped on it (for example 0.50 or 50). This tells you if the engine has been rebored before.
Choose your size
Within limits and not scored: STD (or the size already fitted). Worn or scored: rebore to the next oversize that cleans up the bore.
Oversize chart
Oversizes are added to the standard bore of your model. Older and British bikes often use imperial sizes - these are the metric equivalents.
| Size | Example on a 66.00mm STD bore | Imperial equivalent |
|---|---|---|
| STD | 66.00mm | STD |
| +0.25mm | 66.25mm | +0.010 in |
| +0.50mm | 66.50mm | +0.020 in |
| +0.75mm | 66.75mm | +0.030 in |
| +1.00mm | 67.00mm | +0.040 in |
| +1.50mm | 67.50mm | +0.060 in |
Imperial sizes are rounded (0.010 in = 0.254mm). Always check the exact figure on the product page.
Buy the pistons before the rebore
Your engineer should measure the new pistons and bore the cylinder to suit them, giving the correct piston-to-bore clearance. Send the pistons with the barrel when you book the work.
Shop by part
Found your size? Go straight to the parts.
Size questions
My engine has been rebored before - how do I know the size?
Check the old piston crown for a stamped size such as 0.25, 0.50 or 1.00. Then measure the bore as shown above to confirm it.
Can I fit new rings without reboring?
Yes, if the bore is within the wear limit, is not scored, and the piston is in good condition. The bore should still be lightly honed so the new rings bed in.
What if my bore is beyond the largest oversize?
The cylinder can usually be re-sleeved with a new liner and bored back to standard. See our cylinder liners, or ask us about your model.
Do you rebore cylinders?
Yes - see our precision reboring service.
Still not sure which size?
Send us your bike model and bore measurement and we will tell you which size to order.