PROX Piston Kit for TM RACING EN 250, MX 250 1999-2020

On TM RACING EN 250, MX 250 from 1999 to 2020, the PROX 01.7309.C piston kit (Ø 66.36 mm) fits in place of the original part. At the heart of the top end, the piston soaks up the heat and pressure of every explosion. As it wears, the rings no longer seal against the cylinder: compression drops, power drops with it, and the risk of seizure goes up.

Key features

• Made mainly in Japan, by OEM manufacturers (ProX catalogue).
• Complete kit: piston, rings, pin and circlips.
• Compatible with stock coated (Nikasil) and sleeved cylinders. Not suitable for chrome-plated cylinders.
• Diameter classes to the hundredth of a millimetre for a precise piston-to-cylinder clearance.

Specifications

• Brand: PROX
• Manufacturer part number: 01.7309.C
• Size: C (check that it matches your cylinder)
• Piston diameter: 66.36 mm
• Weight: 340 g
• Engine: 2-stroke

When to replace it?

• Loss of power, sluggish pick-up, harder starting: compression is dropping.
• Piston slap on a cold engine, getting more and more noticeable.
• On teardown: vertical scoring on the skirt, stuck or worn rings, thick carbon on the crown.

Replace the piston as set out in your bike's service manual, sooner if you race, and straight away as soon as these symptoms appear. As a Cross-Mania guideline, plan for a new piston every 20 to 40 hours of engine running depending on whether you race or ride for fun. Always measure the cylinder at the same time: a new piston in an oval cylinder won't last.

Fitting tips

1. Clean the cylinder and check it: diameter, ovality, taper and condition of the bore coating.
2. Check the end gap of each ring in the cylinder against the figures in the manual or the manufacturer's instructions.
3. Fit the rings with the mark facing up and each gap correctly located on its locating pin.
4. Check the small-end needle bearing and the pin: at the slightest play, tight spot or marked needle, replace the bearing.
5. Fit the piston with its arrow pointing to the exhaust side, oil the pin and the small end, then fit the circlips without distorting them, gap opposite the notch. A circlip that has been taken out is replaced with new piston pin circlips, never refitted.
6. Reassemble with a new top end gasket kit and tighten the cylinder head in a criss-cross pattern to the manufacturer's torque.
7. Refill the coolant and fit a new spark plug.
8. Break the engine in: warm it up gently, vary the revs and avoid sustained full throttle for the first hour.

Why PROX?

ProX Racing Parts has been supplying engine and clutch parts to motocross riders since 1975, from its headquarters near Amsterdam. According to its catalogue, most of its parts are made in Japan, by manufacturers that also produce parts for the Japanese motorcycle makers.

Spare parts and items to plan

• Piston rings : VERTEX 53010006640N8 · VERTEX 53010006640
• Top-end gasket kit : VHM SK33000.VITON · VHM SK33000
• Small end needle bearing : NTN
• Coolant : MOTUL 1 L · MOTUL 20 L
• 2-stroke oil : LIQUI MOLY 1 L 3065 · LIQUI MOLY 1 L 3063

Frequently asked questions

Does the kit include the gaskets?
No: the kit contains the piston, rings, pin and circlips. Add a top end gasket kit, because a head or base gasket can never be reused.

Can I replace just the rings later?
Yes, as long as the piston and cylinder are still within tolerance. Replacement rings can be ordered on their own, as can the piston pin and the circlips.

Should I replace the small-end needle bearing?
It isn't included. Check it at every piston change and replace the needle bearing at the slightest play or tight spot.

Compatibility

Check the exact model and year of your bike in the list below before ordering.

Fits:
TM RACING EN 250 2020
TM RACING EN 250 2019
TM RACING EN 250 2018
TM RACING EN 250 2017
TM RACING EN 250 2016
TM RACING EN 250 2015
TM RACING EN 250 2014
TM RACING EN 250 2013
TM RACING EN 250 2012
TM RACING EN 250 2011
TM RACING EN 250 2010
TM RACING EN 250 2009
TM RACING EN 250 2008
TM RACING EN 250 2007
TM RACING EN 250 2006
TM RACING EN 250 2005
TM RACING EN 250 2004
TM RACING EN 250 2003
TM RACING EN 250 2002
TM RACING EN 250 2001
TM RACING EN 250 2000
TM RACING EN 250 1999
TM RACING MX 250 2020
TM RACING MX 250 2019
TM RACING MX 250 2018
TM RACING MX 250 2017
TM RACING MX 250 2016
TM RACING MX 250 2015
TM RACING MX 250 2014
TM RACING MX 250 2013
TM RACING MX 250 2012
TM RACING MX 250 2011
TM RACING MX 250 2010
TM RACING MX 250 2009
TM RACING MX 250 2008
TM RACING MX 250 2007
TM RACING MX 250 2006
TM RACING MX 250 2005
TM RACING MX 250 2004
TM RACING MX 250 2003
TM RACING MX 250 2002
TM RACING MX 250 2001
TM RACING MX 250 2000
TM RACING MX 250 1999