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M21 Inner primary cover seals etc #794678 01/03/20 5:21 am
Joined: Dec 2014
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Howley Online Content OP
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Hi all, happy new year.

I'm piecing together the primary drive on my 1949 M21. I've got a Burton Bike Bits 4 spring clutch kit to go on.

Coming out from the final drive sprocket nut, my parts book shows:

| Chaincase Sliding Plate | Felt Washer | Cork Oil Retaining Washer | Clutch Sleeve Collar | Clutch Sleeve |

Long story short, I'm missing the parts in bold, and the inside diameter of the Chaincase sliding plate is all chewed out. I'm planning to make the missing parts but I'm unsure of how they fit around the sliding plate.

1. Does the felt washer fit tightly around the mainshaft?
2. Does the cork washer fit tightly around the mainshaft?

3. How does the felt washer outside diameter relate to the sliding plate inside diameter?
4. Does the felt washer thickness match that of the sliding plate?
5. How does the cork washer outside diameter relate to the sliding plate inside diameter?

6. Is the felt washer stepped to fit both inside and over the sliding plate?
7. Does the cork washer fit over the sliding plate?

8. Do any of these parts turn inside the sliding plate in such a way that the sliding plate inside diameter would need to be smooth?

Any answers or hints would be much appreciated.

Thanks!

Luke

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Re: M21 Inner primary cover seals etc [Re: Howley] #794729 01/03/20 11:40 pm
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chaterlea25 Offline
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Hi Luke,
When fitting the later 4 spring clutch things change a bit
With the single spring clutch the first cork washer/sleeve (37) that sits against the sprocket sleeve and inside the shaft adaptor
is not used when fitting the 4 spring clutch
On the six spring a smaller cork seal is placed between the clutch adaptor and the sprocket sleeve to provide a rudimentary seal (29)
I cannot see any cork seal being fitted in the relevant 4 spring clutchparts book. ??
You can look theough the BSA books online here
https://www.baxtercycle.com/parts/parts-books-online/bsa/
I do not know if the M gearbox sprocket nut is the same as the A and B series ? on those you can buy a nut with inbuilt oil seal from SRM and others

Pattern sliding plates are readily available but they have two issues at least,
1, the centre hole is not an accurate fit onto the scroll on the back of the shaft adaptor
2, the two parts are rivited together and oil leaks out between the two, a run of sealant around the V gap should seal this..
The large felt disc fits between the primary case and inner primary clamped in place by the sliding plate
Special shouldered bolts hold the sliding plate in place but allow it to move when the gearbox moves for primary chain adjustment
Some people fit an other felt washer between the shaft adaptor shoulder and the sliding plate to help the scroll work ???

You will more than likely have primary chain misalignment after fitting the 4 spring clutch!!
You will probably have to space out the cush drive assembly to compensate and this can make fitting its nut a problem
The nut needs to be fully tightened to 65ft/lbs no matter what

John

Re: M21 Inner primary cover seals etc [Re: Howley] #794758 01/04/20 10:08 am
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Howley Online Content OP
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Thanks for the info John.

So from your critiques of the pattern sliding plates I take it the scroll of the adapter should sit INSIDE the sliding plate hole with a close fit? How much of it should be inside?

From my initial fit-up (clutch placed on by hand and not driven on) the adapter is about 2mm from bottoming out on the mainshaft shoulder and the scroll is still clear of the hole. Adding a second felt should help this?

As an aside, I take it the bolts holding the sliding plate should not be fully tightened?

Re: M21 Inner primary cover seals etc [Re: Howley] #794789 01/04/20 7:42 pm
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chaterlea25 Offline
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Hi Luke,
Before going any further have a look at how the sprockets are lining up with the clutch fitted ?
There were different depth taper adaptors for single and twin BSA's the singles being deeper so the clutch sat farther towards the gearbox
I have not done the swap on an M20 but have done a couple of c1950 B33's, some years ago now though
The sliding plate needs to sit most of the way onto the scroll, half way at least
You may have to space out the primary case from the engine ???
The shoulder on the correct bolts allows the nuts to be tightened and also let the plate slide (stiffly)

If it was easy everyone would do it
John

Re: M21 Inner primary cover seals etc [Re: Howley] #794805 01/04/20 11:42 pm
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Howley Online Content OP
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Thanks again John.

I may as well show you what I've got.

Mainshaft, blue tape shows depth to shoulder on clutch adapter:
https://photos.app.goo.gl/YTLnL85dUoGYEWk1A

Clutch fitted but not driven on, note blue tape:
https://photos.app.goo.gl/5rgAEQLWfCFs2jRp9

Alignment to drive sprocket with NO distance pieces behind it (I have none, some on order):
https://photos.app.goo.gl/Dh4gvDQrmZCmfNXp7

Drive nut engagement with drive sprocket aligned (new nut on order!)
https://photos.app.goo.gl/1SpgKRUXx6uQKHi59

Looks like I'll need about 5mm/0.2" of shims on the drive sprocket.

My sliding plate is too messed up to indicate anything valuable.
Does the fit up and alignment above look OK to you?

How would you recommend spacing out the inner primary cover? Just add multiple cork gaskets?

Luke

Re: M21 Inner primary cover seals etc [Re: Howley] #794884 01/05/20 7:42 pm
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chaterlea25 Offline
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Hi Luke,
Any shims or spacers need to be fitted inside the cush drive splined centre to avoid any relative movement against the shims
On the B33's I machined up new sleeves (65-2528) that fit between the crank bearing and cushdrive sleeve once the overall thickness
had been sorted,.
You would need to fully tighten the clutch centre nut first to finalise the clutch position (make sure you have a puller to remove it )
I finalise the alignment without the inner case fitted as it provides space to use a straight edge
I would make an aluminum spacer to fit between the crankcase mounting and the inside of the inner case once the required thickness has been found
Make a longer footrest bar spacer sleeve (that fits between the engine plate and the inner primary) to compensate for the extra spacer at the front
The thickness of the spacers should be so that the scrolled part of the clutch adaptor fits into the sliding plate comfortably

I do not use the lockwasher on the cushdrive nut, degrease the threads and use some threadlock
This tool can be used to tighten the nut, or a suitably sized socket cut to fit the nut
https://www.eBay.co.uk/itm/Lower-Ball-Joint-Socket-35mm-Screw-Type-For-Peugeot-Citroen-Castle-Socket/161272516269?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

John


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