Gearbox main shaft

[chaterlea25]

Hi Craigy,
I very much doubt that there could be 1cm between the chain and case
A few mm would be fine

There should not be any great pressure on the crank nut unless the wrong spring has been fitted, the correct spring has 3 and 1/4 turns to the best of my knowledge.

John

[jhg1958]

The Cush drive is strong but very short. I have had mine off this year and by the time you are on the last few threads all tension has gone. I used a c spanner on mine but there is a proper tool that is far easier to use.
You will need to lock the engine so leave the clutch and primary drive chain on.

When I replaced the Cush drive nut it was a bit of a bugger so I put copperslip on the thread. But I read that these nuts work loose to grind their way into the cover. So I took it off again cleaning the threads and putting lock tight on it. Do not forget the split pin.

John

[Swarfcut]

  Craigy....
Plenty on the Forum about the cush assembly. Without the spring the big nut should bottom on the end of the drive sleeve, which itself should sit just proud of the crank splines. This is essential to secure the main bearing inner race against the crankshaft cheek. All outlined  on the Forum.

  Over the years BSA had several variants of the sleeve and spring, all of which fit the crank and so a possibility of incorrect model year parts. Your bike should have a spacer behind the drive sleeve, this runs on the oilseal in the crankcase. Circular shims between spacer and sleeve are used as required to correct chain alignment with the clutch chainwheel. Earlier version has a drive sleeve with integral spacer to run on the seal.

 Recommended  torque for the nut is 65 -70 FT/Lbs, bloody tight. Split pin ends up above nut face, and in practice its function is to prevent a loose nut machining a hole in the primary case.

 Drags online parts diagrams show what's what, SRM make a somewhat improved version of the nut.  Tab washers considered ineffective, Loctite favoured these days, as mentioned.

 Swarfy.

[Craigy]

Cheers Swarfy (and everyone), I’ve had a read of a few old posts and think I’ve got my head around it a bit more. One of far more knowledgeable guys from the club is willing to give direction so that’s a big relief.
I just hope it fixes it, I’m desperate to get it out, I love riding it

[Craigy]

Well here’s the latest in the continuing saga……we’ve managed to get everything off on the primary case side to allow access to the drive sprocket. It was slightly loose. It’s been tightened as much possible, but this has only given a couple of mm clearance between the chain and rear of primary case.
I can’t see anyway of getting more clearance…..I was thinking maybe machining a little off the collar but this would put the chain out of alignment to rear sprocket.
Any ideas?

[Swarfcut]

 I'd suggest starting with the rear wheel and making sure the line of the drive chain is in line with the gearbox sprocket, then add the clutch and primary chain and see how this lines up. No need to tighten the clutch nut, a light nip will do for now. Chaincase can be left off to enable a good looky.

 Somewhere on the forum this has been done in great detail, from memory Chaterlea John did some measuring. Maybe the authors can post a link?

 As stated, the primary inner can flex, so worth checking it is straight, a homespun spacer or washers are often necessary to bridge the gap between the frame lug and the lower mounting down by the S/A pivot. The case should not be under any strain.

 Any mis alignment in the front downtubes, mounting lugs and engine plates can result in a twist to the crankcase, so it is better to leave mountings slightly loose, get things aligned, then tighten the fixings.

 Swarfy.

[Craigy]

Swarfy, you could be onto something there…
Should the case distance collar (42-4792) be on the backside of the rear casing? Mine is on the front pushing the case towards the gear. If it was at the rear that would give me the 10mm clearance I’ve seen on other bikes

[Craigy]

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[Craigy]

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