Replace the output shaft seal

The output shaft (countershaft) is the gearbox shaft that carries the front sprocket. A rubber lip seal stops engine oil escaping round it. A worn seal throws oil onto the chain, the swingarm and sometimes the rear tyre. The seal often fails because of a worn shaft, a loose sprocket or grit packed behind the sprocket cover. This is workshop work on most bikes because the seal sits in the crankcase and a damaged seat means engine work.

Safety

• Oil on the rear tyre or rear brake causes a fall. Clean the tyre, rim and brake with brake cleaner, and replace oil-soaked pads before riding.
• Keep fingers away from the chain and sprocket if the engine is running.

What you can check safely

1. Remove the front sprocket cover (a few screws on the left of the engine).
2. Scrape out packed mud and chain grease. Look at the seal behind the sprocket with a torch.
3. Confirm oil comes from the seal lip and not from a cover gasket above it or the gear lever shaft seal.
4. Grip the sprocket and try to move it in and out and up and down. Play here means a worn shaft bearing or a loose sprocket: tell the workshop.
5. Check the sprocket bolt or nut and lock plate are tight. K-Light 125: driving (front) sprocket bolt 10 to 16 N·m.
6. Check the level of engine oil with Check the engine oil level. Overfilling can push oil past a weak seal.

What the workshop does

• Slackens the chain and removes the front sprocket. See Check and adjust chain slack for setting it afterwards.
• Removes any spacer or collar on the shaft and checks it for grooves. A grooved collar is replaced with the seal.
• Lifts out the old seal without scratching the shaft or the seal seat.
• Cleans the seat, oils the new seal lip and drives it in square with a driver that presses only on the outer edge. The K-Light manual says crankcase seals are never reused.
• Refits spacer and sprocket with a new lock plate, torques the bolt, sets the chain slack and checks oil level.
• If the shaft bearing has play or the seat is damaged, the job becomes a crankcase repair: see Assess bottom end and internal engine damage.

What to ask for

• A new seal and a new collar if grooved.
• A check of chain and sprockets while the sprocket is off: Inspect sprockets.

Check your work

• After a ride the area behind the sprocket stays dry.
• Chain slack is correct and the chain runs straight.

If it did not fix the problem

• Leak comes back fast: shaft bearing play or a breather fault raising crankcase pressure. See Check the crankcase breather.

Reference values (K-Light 125)