Crankshaft & conrod assembly

How it works

The crankshaft turns the pistons' up-and-down motion into rotation. Each piston is linked by a connecting rod to a crankpin (an offset journal); when combustion pushes the piston, the rod turns the crankshaft, which then drives the gearbox through the clutch, as well as the alternator and often the timing drive. The crankshaft runs in bearings: pressure-fed shell bearings on most 4-strokes, roller or ball bearings on many 2-strokes and small singles. It is a very strong part that usually lasts as long as the engine if lubrication is right. Problems mostly come from lack of oil or oil pressure: the shells wear and a deep knock appears that grows under load (the classic big-end knock). On a 2-stroke, worn main bearings give a dull rumble and vibration. A damaged crankshaft means fully opening the engine: one of the heaviest repairs on a motorcycle.

Maintenance tips

• ✓The best protection for the crankshaft is the right oil, changed on time, at the correct level.
• ✓A deep knock that grows when you accelerate under load means stop the engine: have it diagnosed before riding.
• ✓Never ride with the oil-pressure warning lit.
• ✓Let the engine warm up before using high revs.
• ✓Any crankshaft work means splitting the bottom end: leave it to a workshop.