The cam chain drives the camshaft from the crankshaft. As it wears, it gets longer at the joints. The valve timing drifts, the tensioner runs out of travel and the chain starts to rattle. A worn chain can jump a tooth (valves hit the piston) or saw into the case and fill the oil with metal. This page helps you decide whether the chain needs replacing, and what a good workshop will do. On most engines it is a top end job; on some it means splitting the crankcase.
Safety
- Do not keep riding with a loud cam chain rattle that adjustment does not cure. A chain that jumps or breaks bends valves and can wreck the head.
- Valve timing must be exactly right before the engine is turned with the starter. A mistake bends valves.
What you can check at home
- Listen: a rattle or clatter from the top of the engine at idle that gets quieter as revs rise is typical. See Cam chain rattles.
- Do the tensioner check in Check the cam chain and tensioner. If the tensioner is at the end of its travel, or the noise stays after adjustment, the chain is worn out.
- Look at the drained oil for bright metal flakes: Inspect engine oil.
- Find out from the manual whether the chain runs on the side of the engine or in the middle between cylinders, and whether the bottom sprocket is part of the crankshaft.
What the workshop does
- Sets the engine at TDC on the compression stroke and photographs and paints the cam timing marks before anything is removed.
- Removes the tensioner, the cam cover and the camshaft sprocket.
- Side chain, sprocket reachable: lifts the old chain off and fits a new endless chain, the type the maker uses.
- Chain in the middle of the engine (some twins and fours): cuts the old chain, hooks the new chain to it with a link and pulls it through by turning the engine slowly by hand, then joins the new chain with a master link riveted with a proper cam chain riveting tool. A clip-type link is less reliable and is avoided where possible.
- Checks the guides, the tensioner blade and the sprockets. A worn chain usually means worn sprockets and guides. Where the bottom sprocket is pressed on or machined into the crankshaft, replacing it is a bottom end job and is quoted separately.
- Sets the cam timing to the marks: Set cam timing after head or cam work.
- Fits the tensioner in its loosest position first, then releases it. A tensioner fitted already extended overtightens the chain and can break.
- Turns the engine slowly by hand at least two full turns and rechecks every mark before starting it.
What to ask for
- A new chain, and the price for guides, tensioner and sprockets if they are worn. Replacing the chain alone on worn sprockets shortens its life.
- Confirmation that the valve timing marks were checked after the tensioner was fitted.
- The old chain back, to see the wear.
Do not try this at home unless
You have the manual, the correct riveting tool and the time to check every mark more than once. Adding a stronger tensioner spring or locking the tensioner tighter to stop the noise is only a short-term measure: too much tension drags on the chain and holds the engine back, and the worn chain is still there.
Check your work
- No rattle at idle when warm.
- The engine starts and idles normally, with compression as before.