The chain adjusters move the rear axle backward or forward in the slots of the swingarm to set chain slack. Common types: a bolt and lock nut pushing on an axle block at each side; a round snail cam on each end of the axle; or a block in the swingarm end. Marks on each side of the swingarm show the axle position.
What it does
The adjusters set chain slack and keep the rear wheel in line with the front sprocket. Both sides must be set to the same mark. If one side is further back, the wheel sits at an angle, the chain wears on one side and the bike can pull to one side.
Signs it is worn or failing
- Axle at the end of the adjustment range: see Chain adjuster is at the end of its range. This usually means the chain is worn, not the adjuster.
- Adjuster bolt seized, threads stripped or lock nut missing, so the axle moves when the nut is tightened.
- Marks unreadable or not matching what you see when you look down the chain.
- Snail cam with worn notches that do not hold position.
When to replace it
Replace bent, stripped or seized adjuster bolts, blocks or cams. A replacement chain is the cure when the adjuster has run out of travel.
What to match when buying
- Thread size and length of the bolt, and the block or cam for your swingarm.
- Snail cams are made in pairs for one model; fit both new.
Fitting notes
- Set both sides to the same mark, then sight along the chain from behind the sprocket. It should run straight off the rear sprocket. If the marks look wrong, trust the chain line and a measured distance from swingarm pivot to axle on both sides.
- Clean and grease the adjuster threads at each chain service so they move freely.
- After setting slack, tighten the axle nut to the manual torque and check the rear brake. See Check and adjust chain slack.