A top end rebuild needs the right piston kit for your model and year, the right size for your cylinder and a matching gasket kit. For parts, use our bike finder or browse piston kits.
When does a top end need doing?
Go by your owner's or service manual, which for competition MX and enduro bikes usually gives a piston replacement interval in engine hours. In general:
- Two-stroke: the piston, rings and small end bearing are routine wear parts, and with no valve train or cam timing to set, the job is relatively simple.
- Four-stroke: a piston change also involves the head, cam timing and valves, and the manual calls for regular valve clearance checks in between.
Racing, sand, mud and hard enduro wear an engine faster than trail riding, and a poorly kept air filter lets in dust that quickly wears pistons and rings (see our air filter chart). An RFX wireless hour meter makes intervals easy to track. Signs a top end is due include lost power, harder starting, new noises from the cylinder and, on a four-stroke, rising oil use.
Our piston kit range
We have 142 piston kit listings, plus one complete top end kit:
| Brand | Listings | What they cover |
|---|---|---|
| ProX | 123 | 57 two-stroke and 59 four-stroke bikes and quads; 7 watercraft and scooters |
| Hi Level | 11 | Mainly older road, trail and small capacity models |
| Vertex | 4 | KTM and Kawasaki four-strokes, plus a KTM 125 SX complete top end kit |
| TourMax | 4 | Older Yamaha and Honda road and trail models |
Piston sizes and grade letters
Many MX and enduro cylinders are plated rather than sleeved. A plated bore is not bored oversize, so pistons come in closely spaced grades, marked by letter, to give the clearance the manufacturer specifies. Of our 123 ProX listings, 111 are letter graded, with neighbouring grades typically 0.01 mm apart. The ProX kit for the Yamaha YZ250 from 1999, for example, comes in 66.35, 66.36, 66.37 and 66.38 mm: grades A, B, C and D.
Look for a grade mark on the cylinder or old piston, and measure the bore against your manual. A scored or worn plated bore needs replating or replacing; a new piston alone will not cure it.
The other 12 ProX listings are sized in standard and oversize steps rather than grades, which suits cylinders with a liner that can be rebored (a few are listed in one size only). The ProX PW50 piston kit, for example, is offered at 40.00, 40.50 and 41.00 mm. Buy the piston before the cylinder is bored, so the bore can be finished to suit it.
What comes in a kit
Contents vary by brand. The manufacturers state:
- ProX piston kits: piston, ring set, gudgeon pin and circlips.
- Vertex piston kits: piston, rings, gudgeon pin and circlips.
- The Vertex top end kit for the KTM 125 SX 2016-2022: piston, rings, circlips, gudgeon pin, base gaskets, head O-rings and, where applicable, the small end bearing.
Hi Level and TourMax listings do not list the contents, so email us to confirm them. Vertex ring sets are also sold on their own in piston rings.
Four-stroke kit titles usually show the compression ratio, such as 13.8:1 on the ProX kit for the 2019-2026 Yamaha YZ250F. High compression versions are marked High Comp or HC. Stick with the standard ratio unless you have a reason to change.
Top end gasket kits
Fit new gaskets whenever the head or cylinder comes off, as used ones cannot be relied on to seal. We list 110 top end gasket kits from ProX and Vertex. Vertex says its kits include the head, base, exhaust and, where fitted, power valve gaskets, but not formed rubber valve cover gaskets. Most are in top end gaskets and seals; for bigger jobs, see complete gasket sets.
Checks while it is apart
- Fit new circlips, seat them fully, and cover the crankcase opening so nothing can drop in.
- Check ring end gaps in the bore against the manual, and fit rings with any markings facing up.
- On a two-stroke, inspect the small end bearing and check the con rod for play; see con rod kits.
- On a four-stroke, check the valves and valve clearances; see engine valves.
- Torque fasteners to the figures and sequence in the manual, then follow its running in procedure.
Matching piston and gasket kits for common models
Years shown are where both listings cover the model, going by each title and fitment table; check both before ordering.
| Bike | Years | Piston kit | Gasket kit |
|---|---|---|---|
| Yamaha YZ125 | 2022-2026 | ProX piston kit | ProX gasket kit |
| Yamaha YZ250 (two-stroke) | 2002-2024 | ProX piston kit | Vertex gasket kit |
| Yamaha YZ250F | 2019-2026 | ProX piston kit | ProX gasket kit |
| KTM 85 SX | 2018-2025 | ProX piston kit (grade D: here) | ProX gasket kit |
| KTM 250 SX-F | 2023-2026 | ProX piston kit (grade C: here) | ProX gasket kit |
| Honda CRF250R | 2020-2024 | ProX piston kit | Vertex gasket kit |
| Honda CRF450R | 2023-2026 | ProX piston kit | ProX gasket kit |
| Kawasaki KX250 (four-stroke) | 2023-2024 | ProX piston kit | ProX gasket kit |
| Suzuki RM-Z450 | 2013-2026 | ProX piston kit | ProX gasket kit |
| Beta RR 300 (two-stroke) | 2018-2021 | ProX piston kit | ProX gasket kit |
How to find a piston kit for your bike
- Use our bike finder or models hub, or browse piston kits and the engine top end range.
- Check the fitment table for your exact model and year and, on a four-stroke, the compression ratio.
- Pick the size: your grade letter for a plated bore, or the rebore size for a sleeved one. If your grade is missing from the size menu, search for your model or email us, as a few grades are listed on their own.
- Add the matching top end gasket kit.
Not sure which kit or grade you need? Email sales@apexpowersports.co.uk with your model, year and any grade mark. Delivery is UK Next Day £7.99, free on orders of £100 or more; see delivery for faster options.