ATHENA cast piston Top End Kit (Piston + Gaskets) for FANTIC XE 125 and YAMAHA YZ 125 2022-2026

On YAMAHA YZ 125 and FANTIC XE 125 from 2022 to 2026, the ATHENA piston + gasket top end kit (Ø 53.95 to 53.97 mm depending on the size class) fits in place of the original part. Rebuilding a top end means replacing the piston and every gasket you have removed. This kit combines them: a complete piston and a top end gasket kit, made for each other.

Specifications

• Brand: ATHENA
• Manufacturer part numbers: P4C0540271014A, P4C0540271014B, P4C0540271014C
• Engine: 2-stroke
• Piston diameter: A = 53.95 mm, B = 53.96 mm, C = 53.97 mm
• Available versions: A: ref. P4C0540271014A; B: ref. P4C0540271014B; C: ref. P4C0540271014C
• Weight: 440 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. Use the gaskets supplied in the kit, fitted dry, 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?

Founded in 1973 near Vicenza, Athena is an Italian engine gasket specialist that has made its gaskets on its own production lines since day one, as well as a renowned manufacturer of pistons and cylinder kits. The brand supplies original equipment to manufacturers such as KTM, Ducati, Fantic, Piaggio and Triumph, and is a partner of the MXGP motocross world championship.

Spare parts and items to plan

• Piston rings : PROX · VERTEX
• Piston pin : VERTEX · WOSSNER
• Piston pin clips : WOSSNER
• Small end needle bearing : HOT RODS · ATHENA
• Spark plug : NGK
• Coolant : LIQUI MOLY 1 L · TWIN AIR
• 2-stroke oil : LIQUI MOLY 1 L · LIQUI MOLY 4 L

Frequently asked questions

What does the kit contain?
The complete piston (rings, pin and circlips) and a top end gasket kit. The small-end needle bearing is not included.

Which size should I order?
Measure your cylinder: A, B, C correspond to different actual diameters (53.95 to 53.97 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:
FANTIC XE 125 2026
FANTIC XE 125 2025
FANTIC XE 125 2024
FANTIC XE 125 2023
FANTIC XE 125 2022
YAMAHA YZ 125 2026
YAMAHA YZ 125 2025
YAMAHA YZ 125 2024
YAMAHA YZ 125 2023
YAMAHA YZ 125 2022