VERTEX Racing Forged Piston for SUZUKI RM 125 1990-1999

On SUZUKI RM 125 from 1990 to 1999, the VERTEX Racing forged piston (Ø 53.95 to 53.98 mm depending on the size class) fits in place of the original part. The piston takes every combustion and passes it on to the connecting rod. Forged rather than cast, it stands up better to thermal shock and high revs: a real asset for an engine pushed hard in racing or tuned.

Specifications

• Brand: VERTEX
• Manufacturer part numbers: 22551A, 22551B, 22551C, 22551D
• Compression height: 29.6 mm
• Piston pin diameter: 15 mm
• Rings: 1 x 1 mm
• Piston diameter: A = 53.95 mm, B = 53.96 mm, C = 53.97 mm, D = 53.98 mm
• Available versions: A: ref. 22551A; B: ref. 22551B; C: ref. 22551C; D: ref. 22551D
• Engine: 2-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.

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 VERTEX?

Founded in Italy in 1953 as a workshop making parts for agricultural engines, Vertex now makes its entire production in Cadelbosco Sopra, near Reggio Emilia, on CNC machines. Since 1996 Vertex has been KTM's official original-equipment piston supplier: its pistons are fitted as standard to many KTM, Husqvarna and GasGas bikes.

Spare parts and items to plan

• Piston rings : WOSSNER · VERTEX
• Piston pin : WOSSNER WP032 · WOSSNER CW15
• Piston pin clips : VERTEX · WOSSNER
• Top-end gasket kit : ATHENA · PROX
• Small end needle bearing : PROX · NTN
• Spark plug : NGK 4492 · NGK 3230
• Coolant : LIQUI MOLY 1 L · TWIN AIR
• 2-stroke oil : LIQUI MOLY 1 L · MOTUL 800 1 L

Frequently asked questions

Does the piston come complete?
Yes: piston, pin, circlips and ring(s). The top end gasket kit has to be ordered separately, because a head or base gasket can never be reused.

Which size should I order?
Measure your cylinder: A, B, C, D correspond to different actual diameters (53.95 to 53.98 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.

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:
SUZUKI RM 125 1999
SUZUKI RM 125 1998
SUZUKI RM 125 1997
SUZUKI RM 125 1996
SUZUKI RM 125 1995
SUZUKI RM 125 1994
SUZUKI RM 125 1993
SUZUKI RM 125 1992
SUZUKI RM 125 1991
SUZUKI RM 125 1990