1975 850 MK3 primary chain chewing into primary

The tensioner holds the chain up or down depending on whether it is being driven or on over-run. If the chain can touch above or below then it is too long...this is probably a chain problem as it's almost impossible on the Mk3 to have the centres too close together.

If your tensioner is really not working then I imagine a chain could destroy itself quite quickly.

If you fill the reservoir with oil and pump the pistons, do they stay expanded ? If not then perhaps there is dirt in the small ball valve.

Also, did you change the engine triple sprocket? I was surprised to find there are different offsets!

I replaced my chain after 36 years (30,000 miles) after it started knocking. The tensioners should prime themselves while in use if the oil is filled to the level plug, on the centre stand. Mine uses ATF (But only on a Commando)

Previously Neil Wyatt wrote:

Also, did you change the engine triple sprocket? I was surprised to find there are different offsets!

I replaced my chain after 36 years (30,000 miles) after it started knocking. The tensioners should prime themselves while in use if the oil is filled to the level plug, on the centre stand. Mine uses ATF (But only on a Commando)

I have just acquired a Commando (850 MkIII) and the seller told me to use ATF in the primary case, I'm in the process of assembling a 99 as well and have primary case oil for it. Why do you only use ATF on the Commando?

ATF can be too thin for the chain tensioner and it loses prime, if you get the rattle of a loose primary on start up then change to the factory 20/50. ATF works well otherwise as its designed for auto boxes with lots of clutches..

Correct John, but as ATF works well in my Mk3 I use it. If it didn't I'd use SAE 20 like all my other bikes use. ATF is thought to be a bit thin for the tin chain cases, though I wouldn't argue with anyone who uses it.

SAE 30 is my alternative if I run out of SEA 20, though the Commando spec for my Mk3, as you say was 20/50.

Like all things, it's what works for you. 'In the light of experience' as they say.

Just one word of warning: ('In the light of experience') never use any oil with the slightest amount of synthetic, in a Norton primary chain case. Never mind clutch slip, the lack of any drive would be nearer the mark!