PROX Piston Kit for HONDA XL 500, XR 500 1978-1985

On HONDA XL 500, XR 500 from 1978 to 1985, the PROX piston kit (Ø 89 to 91 mm depending on the size class) 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 numbers: 01.1509.000, 01.1509.050, 01.1509.100, 01.1509.150, 01.1509.200
• Compression ratio: 8.6:1
• Piston diameter: 89 mm = 89 mm, 89.50 mm = 89.5 mm, 90 mm = 90 mm, 90.50 mm = 90.5 mm, 91 mm = 91 mm
• Available versions: 89 mm: ref. 01.1509.000; 89.50 mm: ref. 01.1509.050; 90 mm: ref. 01.1509.100; 90.50 mm: ref. 01.1509.150; 91 mm: ref. 01.1509.200
• Weight: 480 g
• Engine: 4-stroke

Choosing the right size class

The letters A, B, C… are very close diameter grades, one hundredth of a millimetre apart. Measure the cylinder bore with a bore gauge (diameter, ovality and taper) and choose the grade that gives the piston-to-cylinder clearance specified by the manufacturer. As a rule, grade A goes into a new or reconditioned cylinder, grade B into a cylinder that has already been run in. A scored or out-of-spec cylinder has to be reconditioned: you don't make up for its wear with a bigger grade.

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.
• On a four-stroke: oil consumption, blue smoke and a fouled spark plug.

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 50 to 100 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 the gaps staggered, never letting the ends of the oil control ring overlap.
4. 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.
5. Reassemble with a new top end gasket kit and tighten the cylinder head in a criss-cross pattern to the manufacturer's torque.
6. Refill the coolant, fit a new spark plug and change the oil with fresh engine oil and a new filter.
7. 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 : WOSSNER 890XSY-3 · WOSSNER 895XSY-3
• Top-end gasket kit : CENTAURO · ATHENA
• Spark plug : NGK 6681 · NGK 4929
• Coolant : LIQUI MOLY 1 L · TWIN AIR
• 4-stroke engine oil : LIQUI MOLY 10W40 1 L · MOTUL 10W50

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.

Which size should I order?
Measure your cylinder: 89 mm, 89.50 mm, 90 mm, 90.50 mm, 91 mm correspond to different actual diameters (89 to 91 mm). Choose the class that gives the piston-to-cylinder clearance specified by the manufacturer.

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.

Compatibility

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

Fits:
HONDA XL 500 1982
HONDA XL 500 1981
HONDA XL 500 1980
HONDA XL 500 1979
HONDA XL 500 1978
HONDA XR 500 1985
HONDA XR 500 1984
HONDA XR 500 1983
HONDA XR 500 1982
HONDA XR 500 1981