ATHENA Cast Piston for BETA RR 125 2018-2024

The ATHENA cast piston (Ø 53.95 to 53.98 mm depending on the size class) is designed for BETA RR 125 from 2018 to 2024. A cast piston is made by pouring molten aluminium into a mould. It expands a little less than a forged piston: the engine runs with a tighter piston-to-cylinder clearance and is quieter when cold. It's the right choice for leisure or enduro riding, with regular maintenance.

Specifications

• Brand: ATHENA
• Manufacturer part numbers: S4C05400021A, S4C05400021B, S4C05400021C, S4C05400021D
• Engine: 2-stroke
• Piston diameter: A = 53.95 mm, B = 53.96 mm, C = 53.97 mm, D = 53.98 mm
• Available versions: A: ref. S4C05400021A; B: ref. S4C05400021B; C: ref. S4C05400021C; D: ref. S4C05400021D
• Weight: 170 g

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 ATHENA?

Athena, an Italian company founded in 1973 near Vicenza, designs gaskets, pistons and cylinder kits for motorcycles. An OEM supplier to KTM, Ducati, Fantic, Piaggio and Triumph and a partner of the MXGP world championship, it develops its parts on the test bench and on the track with racing teams.

Spare parts and items to plan

• Piston rings : PROX · VERTEX
• Top-end gasket kit : ATHENA · PROX
• Small end needle bearing : PROX · ATHENA
• Spark plug : NGK 4549 · NGK 7544
• Coolant : LIQUI MOLY 1 L · TWIN AIR
• 2-stroke oil : LIQUI MOLY 1 L · MOTUL 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:
BETA RR 125 2024
BETA RR 125 2023
BETA RR 125 2022
BETA RR 125 2021
BETA RR 125 2020
BETA RR 125 2019
BETA RR 125 2018