Piston

The piston is the round aluminium plug that moves up and down inside the cylinder. It carries the piston rings and is joined to the connecting rod by a steel pin (the gudgeon pin), held by two clips.

What it does

When the mix burns, the pressure pushes the piston down and the connecting rod turns the crankshaft. The piston must stay light, move freely and seal with its rings. It is ground slightly oval so it becomes round when hot. Overheating, a lean mixture or lack of oil can melt the crown or seize the skirt on the bore.

Signs it is worn or failing

• Knock when cold that fades when warm.
• Pinking or a white, blistered plug: a warning that the crown is running too hot.
• Engine seized or low compression.
• Blue smoke when the skirt and ring grooves are worn.

When to replace it

When it is scored, cracked, holed, or past the size limit, or whenever the cylinder is bored oversize. Replace pin clips every time. Follow your bike's manual.

What to match when buying

• Match the engine code and the size: standard or oversize (for example "+0.25"). The size is marked on the crown or the box.
• Buy as a kit with rings, pin and clips matched to that piston.
• Check pin diameter and compression height against the old piston. A wrong height changes compression and can hit the valves.
• Good pistons have clean machining, no casting holes and crisp ring grooves. Very light, rough pistons are a warning sign.

Fitting notes

• Workshop job. Fit with the "IN" or arrow mark the way the manual says (on the K-Light, IN toward the inlet valve).
• Use new clips with the gap away from the slot, and cover the crankcase opening so a clip cannot drop in.
• Measure the skirt at right angles to the pin, at the height the manual gives. See Assess the top end.

Reference (K-Light 125)