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Chain Gauge question

Hello Folks

i set about buying a final drive chain for the M20 as it is extremely worn. It looks like I made the fatal assumption that is would be a standard chain; presumably 5/8" x 1/4". Well, the chain turned up and as ever with this bike it wont fit as the width is significantly narrower, although pitch is same as old one. The chain wont fit on the sprocket either (too narrow).


I happen to have a spare sprocket, a new one and the thickness of this 5.77 mm (not sure what closest imperial is) compared to that fitted to the bike that is 9.21 mm thick; even this is not quite thick enough for the old chain as has quite some sideways movement. The new chain fits by spare sprocket. The old sprocket doesn't look too worn, that's why is was hoping to just fit a chain.

My options are:
1. Use spare (new) sprocket at 5.77 mm thickness with new chain. However, i would probably then need to change gear box sprocket. This is 1 of my questions. Is this a difficult job? It looks like its behind the clutch/primary drive chain housing and might be a mare to get at and need a few special tools? Any comments of what's involved?

2. Send chain back and get a chain suitable for a 9.21 mm sprocket; whatever that is. This would seem to be less work but maybe i should not continue with a non-standard setup anyway, or is this a common deviation and is perfectly fine?


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Re: Chain Gauge question

The correct chains for both on an M20 are 5/8 x 1/4 which is a 520 chain (95 links for the rear chain).

However I see on "The Chainman" site he also lists a narrow 520 for a Norton. I have no idea what that means? I would try your new chain on your gearbox sprocket and if it doesn't fit....send it back!! Ron

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Re: Chain Gauge question

Is this a WDM20?...Ian

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Re: Chain Gauge question

The 'thin' chain for Norton relates to it having old-fashioned Renold side plate thicknesses. Modern chain is too physically wide for the narrow spaces in the Norton chain line even though pitch and roller size are correct.

The 5/8" x 1/4" rear should run on a sprocket with a tooth width of .225" (this is probably the 5.77mm sprocket).

A width of 9.21mm (.363") is probably the .356" tooth required by a 5/8" x 3/8" chain. Did some later models perhaps use a wider chain ?

You'll need to check the front sprocket...It may be that a PO has fitted a wider rear sprocket and chain but left the original front sprocket ? It's likely to be pretty worn if a wider chain has been slopping around on it.

There is no way of avoiding the fact that sorting things out properly will involve dismantling the primary drive.

Re: Chain Gauge question

Hello Rik,

thanks for reply. Yes I had a look at front sprocket to get rough assessment of width and it easily fills the gap in what I suppose is my 3/8" width chain. I did this by looking down the gap under magneto behind gearbox. I can't say how worn it is though as would need to dismantle to get at it. The more I think about it, the more I think it might be best to change the lot to be sure.

You are right the 1/4" chain fits nicely on my 5.77 mm sprocket. Side plate spacing on this chain is 6.42 mm (1/4" !) So has a little clearance for sprocket.

So it looks like although 1/4" width is standard on my M20 (it's not a WDM20), a 3/8" setup must be available. One might suspect a chunkier chain might be longer lasting and more robust but not sure?

So to get at front sprocket I need to take all primary drive off including clutch. It looks like I might need at least 2 special tools; a spring compressor clamp and hub extractor. Do you know if there's a good procedure knocking about to change sprockets?

Cheers Neil

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Re: Chain Gauge question

I can't comment on the specific BSA clutch aspect...but which clutch do you have ?

The WM20 (and most Nortons) were unusual in using the 1/4" chain. Many manufacturers had changed to the wider chains. In theory, the smaller and lighter a good quality chain is, the better in terms of wear to sprockets and clearance to other components. Chains rarely break their side plates. Certainly not with 14 bhp.

As far as I can tell, the M20 carried on with a 1/4" chain. Is it possible that your bike has been fitted with heavier chain and sprockets intended for a twin-cylinder model ?

Re: Chain Gauge question

I don't suppose anyone has a BSA drawing for these sprockets? The material spec. and heat treatment would be handy.

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