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1947 M20 primary drive and clutch questions

Hello folks

Now I have a few clutch tools the strip down of my bike is moving along quite nicely although as ever raises a couple of questions. One is to do with tools and the other is to do with clutch parts.
On the tooling question. Service sheet 610 states that the clutch extractor tool 61-1912 has a pitch of 24 TPI. However, I’ve see elsewhere on this site that this is an error and should be 26 TPI. Can anyone clear up what the pitch of this tool should be?
I ended up taking gamble on this and buying the tool from dragonfly and is a 24 TPI type. It worked ok and screwed in rather loosely for my liking but did not bind suggesting to me it might be 24 tpi after all.

On the parts question, on pulling clutch off intact (still not got the spring tool) I encountered a few surprises. First off there was no cork sleeve seal behind the clutch. Maybe this why I had clutch slip and oil in the clutch. Also, the primary chain case inner looks odd and different to that on the exploded diagram on dragonfly for a B/C/M Rigid. It has no sliding plate or cork washer. Can anyone confirm I am missing parts or is this a different version of the case and all is as it should be. I have attached a few pics to illustrate/

Cheers, Neil





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Re: 1947 M20 primary drive and clutch questions

['First off there was no cork sleeve seal behind the clutch. Maybe this why I had clutch slip and oil in the clutch...']...

That was probably not the issue...About 25 years ago (or more) I had one of the corks sleeves break up...I didn't have a replacement at the time and just reaassembled it all without one...No problems and I've never fitted one since...

It appears you have the wrong inner chaincase...I think it's off a C10/C11/C11G etc...There are many versions of the tin chaincases for different models..Check the part number in the M20 parts list...When looking for a replacement you will find the part number stamped into the casing just above the hole where the garbox mainshaft comes through, at about 1 O'clock when your looking at the inside of the case....Ian

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Re: 1947 M20 primary drive and clutch questions

Correct WM20 / rigid inner chaincase is stamped 66-7602.

Re: 1947 M20 primary drive and clutch questions

Thanks for the replies guys,

Was a bit surprised I don't need Cork sleeve behind clutch I suppose that should be good news in some ways. However, oil has been getting in the clutch but maybe not so much from this direction. The domed clutch cover gasket was very poor so maybe that's where the oil got in.

As noted elsewhere this bike is a bit of a mongrel with a few odd parts. I thing even the gearbox is not standard, as although not used, it has a speedo drive coming out of the casing and 3/8" final drive chain. So... tempted to leave the chain case as is. After al, it fits fine and looks ok.

Cheers Neil

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