The crankshaft is the heavy shaft inside the bottom of the engine, with two round flywheels (webs) and a pin between them. The connecting rod joins this pin to the piston. On most commuter singles the crank and rod are pressed together as one assembly and turn on ball bearings in the crankcase.
What it does
The rod turns the up and down push of the piston into rotation of the crank. The webs carry counterweights to cut vibration. The big end (rod to crank) and main bearings need clean oil under pressure. Dirty or low oil, over-revving or lugging wears these bearings and lets the rod knock.
Signs it is worn or failing
- Deep knocking from the bottom of the engine.
- Rumbling or growling from inside.
- New or increased vibration.
- Metal particles in the oil.
- In the worst case the engine seizes. Stop riding at the first deep knock.
When to replace it
When the big end has radial or side play past the limit, the crank runout is too high, or the main bearings are rough or loose. On many singles the whole crank assembly is replaced rather than rebuilt. Follow your bike's manual.
What to match when buying
- Match engine code, stroke, rod length and the taper and keyway for the flywheel. Cranks for a kickstart and a non-kick engine can differ.
- Buy main bearings and crankcase oil seals at the same time, with a gasket set.
- A good crank is true, with no rust, a smooth big end with no play and clean threads. Ask for runout to be checked on V-blocks before fitting.
Fitting notes
- Workshop job: the crankcase must be split. See Assess bottom end and internal engine damage.
- Find the cause of failure (oil starvation, dirty oil, wrong oil) before fitting a new crank, or it will fail again.
- After the rebuild use clean oil and the right quantity: Change the engine oil.
- Crankshaft bearings are often a tight fit on the shaft. A puller tool pulls the shaft through the bearing so only the inner ring is loaded.
Reference (K-Light 125)
| Item | Value | PDF page |
|---|---|---|
| Big end side clearance | 0.1 to 0.35 mm, limit 0.55 mm | 236 |
| Big end radial clearance | 0.008 to 0.018 mm, limit 0.05 mm | 236 |