PROX Piston Kit for SUZUKI ER 125, TS 125 1978-1987

The PROX piston kit (Ø 57 to 57.25 mm depending on the size class) is designed for SUZUKI ER 125, TS 125 from 1978 to 1987. The piston takes the pressure of every combustion and passes it on to the connecting rod, thousands of times a minute. Over the hours it wears, its rings stop sealing properly and the engine loses compression: it starts less easily, pulls less hard and runs hotter.

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: 01.3005.100, 01.3005.125
• Piston diameter: 57 mm = 57 mm, 57.25 mm = 57.25 mm
• Available versions: 57 mm: ref. 01.3005.100; 57.25 mm: ref. 01.3005.125

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.

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. 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. Break the engine in: warm it up gently, vary the revs and avoid sustained full throttle for the first hour.

Why PROX?

Founded in 1975 and based near Amsterdam, ProX specialises in engine and clutch wear parts for off-road bikes. The brand has most of its range made in Japan, by OEM suppliers, for quality on a par with the original parts.

Spare parts and items to plan

• Piston rings : see the selection
• Piston pin : WOSSNER CW14 · WOSSNER CW16
• Top-end gasket kit : ATHENA · CENTAURO
• Small end needle bearing : see the selection
• Spark plug : NGK 2411 · NGK 2912
• Coolant : LIQUI MOLY 1 L · TWIN AIR
• 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.

Which size should I order?
Measure your cylinder: 57 mm, 57.25 mm correspond to different actual diameters (57 to 57.25 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:
SUZUKI ER 125 1981
SUZUKI ER 125 1980
SUZUKI ER 125 1979
SUZUKI TS 125 1987
SUZUKI TS 125 1986
SUZUKI TS 125 1985
SUZUKI TS 125 1984
SUZUKI TS 125 1983
SUZUKI TS 125 1982
SUZUKI TS 125 1981
SUZUKI TS 125 1980
SUZUKI TS 125 1979
SUZUKI TS 125 1978