Burman gearbox - AMC Clutch

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Burman gearbox - AMC Clutch

Postby Pharisee » Tue Aug 22, 2017 11:27 am

Has anyone successfully mated an AMC, 3 spring clutch with rubber cush drive to an earlier Burman gearbox? I've found a brief document in the archives that suggests that the only 'modification' necessary is the addition of a 3/8" wide spacer on the gearbox mainshaft behind the clutch.
I just know that somebody is going to ask "Why?" so I'll give you my reason. My latest 'basket case' restoration has a '54 Burman GB type gearbox but I don't have a Burman clutch. What I do have is an almost perfect, complete AMC clutch from a later '60 G80. I've over-budget on this project now and in the interests of keep cost even remotely sensible, I'd like to avoid having to buy a complete Burman clutch.

I said it was 'almost' perfect... The cush drive rubber segments are a bit, well, squishy. If I dig them out, am I going to have great difficulty in putting new ones back in? It's not a job I've ever tackled so I'm a bit of a 'segment virgin'!!

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Re: Burman gearbox - AMC Clutch

Postby clanger9 » Tue Aug 22, 2017 2:11 pm

I can't help with the AMC clutch question, but if you need to replace the clutch rubbers, then I got some very helpful advice in this thread:
viewtopic.php?f=15&t=16226

It looked impossible, but turned out not to be as difficult as feared....
1966 AJS Model 14 250 CSR "cafe racer"
1993 Ducati 750SS

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Re: Burman gearbox - AMC Clutch

Postby zwarts » Tue Aug 22, 2017 3:51 pm

Yes it is quite possible if your Burman is a B52 box. I ran my '39 sprint bike with an AMC clutch/B52 for many seasons without any problems. From memory, I made the 3/8" spacer from an old (Ariel) main shaft adaptor. I recall that the tin chain case required removal of the oil lip behind the clutch to get enough clearance. The spline form for the B52 and the AMC main shafts is almost identical, which is no surprise as the AMC box is a development of the earlier Norton boxes and they were made by Burman from the early 1930's!

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Re: Burman gearbox - AMC Clutch

Postby Pharisee » Tue Aug 22, 2017 7:25 pm

zwarts wrote:Yes it is quite possible if your Burman is a B52 box. I ran my '39 sprint bike with an AMC clutch/B52 for many seasons without any problems. From memory, I made the 3/8" spacer from an old (Ariel) main shaft adaptor. I recall that the tin chain case required removal of the oil lip behind the clutch to get enough clearance. The spline form for the B52 and the AMC main shafts is almost identical, which is no surprise as the AMC box is a development of the earlier Norton boxes and they were made by Burman from the early 1930's!


Yep... It was a 3/8" thick spacer that was mentioned in the info I got from Christian's Archive. The AMC clutch hub is a good sliding fit on the Burman mainshaft. Would you know if the thread for the securing nut is the same on both? It looks like the normal large nut usually fitted to the Burman clutch will be too big for the AMC hub so I may well have to source a smaller nut.

clanger9 wrote:I can't help with the AMC clutch question, but if you need to replace the clutch rubbers, then I got some very helpful advice in this thread:
viewtopic.php?f=15&t=16226

It looked impossible, but turned out not to be as difficult as feared....


Hmmm.... I may be in trouble here as the clutch is 'on the bench' and I don't have an old mainshaft. If anyone has an old mainshaft laying about and they don't mind it being held in a vice, if they could see their way clear to lending it to me, I'd very much like to borrow it for a few days :D

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Re: Burman gearbox - AMC Clutch

Postby zwarts » Tue Aug 22, 2017 9:00 pm

Yes, the main shaft thread is the same dia. and pitch on the Burman and AMC boxes. You need to use the AMC nut, as you've already discovered! I would also recommend fitting a Nordlock washer to retain the nut.

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Re: Burman gearbox - AMC Clutch

Postby Pharisee » Tue Aug 22, 2017 9:40 pm

zwarts wrote:Yes, the main shaft thread is the same dia. and pitch on the Burman and AMC boxes. You need to use the AMC nut, as you've already discovered! I would also recommend fitting a Nordlock washer to retain the nut.


Thanks for the thread information. I'd already ordered an AMC nut and standard spring washer on spec. but I'll take your advice and I've now ordered a couple of M16 Nordlock washers.

I've also cut a piece of 3/16" thick "Usaspead" high carbon steel gauge plate that I can hold in the vice and the clutch hub is a good fit over it. It only holds in two of the splines, of course, but hopefully it will be strong enough to stop the centre of the hub turning while I apply some torque to the outer hub with a clutch locking tool. Hopefully, I can get it all re-assembled without the loss of too much knuckle skin!!

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Re: Burman gearbox - AMC Clutch

Postby zwarts » Wed Aug 23, 2017 10:29 am

John, there are two other things which are worth considering when doing this conversion.
The 3/8" spacer is a nominal figure and you may have to adjust the thickness to get the chain alignment spot on.
Using the AMC clutch will alter the overall gearing as it has 42 teeth, as opposed to 40 teeth on the Burman clutch sprocket. If you have the std. engine sprocket (21 t for the 500) and the std. gearbox sprocket (16 t), this will reduce top gear ratio from 5:1 down to 5.25:1. As the AMC singles are already pulling a slightly lower top gear ratio than most of the comparable singles of the era, it is preferable to use this opportunity to gear up slightly. As we are stuck with 42t on the rear wheel and the largest engine sprocket (with the transmission shock absorber) is 21t, this only leaves the gearbox sprocket to play with. A 21t engine with 17t gearbox sprocket gives a top gear ratio of 4.94:1. This will be fine. Alternatively, depending on availability, a 20t engine sprocket with an 18t gearbox will give 4.9:1 top gear ratio. If the club spares don't have alternative gearbox sprockets for the Burman box, try Draganfly. They have a good stock of Burman parts.
Cheers, Bob.

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Re: Burman gearbox - AMC Clutch

Postby Pharisee » Wed Aug 23, 2017 4:14 pm

Bob...
Thanks for the information. Not having a Burman clutch to hand, I hadn't realised that there was a difference in the number of teeth.
First off... The basket case I'm re-building is a 1954, 350cc G3/LS.
As you rightly say, the AMC clutch basket and the rear wheel sprocket are both 42 teeth.
The final drive sprocket currently fitted to the gearbox is 16 teeth. The gearbox is also currently 'on the bench' so it's not difficult to change if necessary.
Among the bits I acquired was a brand new engine sprocket / dynamo drive sprocket. The primary drive sprocket is 18 teeth and the dynamo sprocket is 21 teeth.
At the moment, all the sprockets are accessible and could easily be changed. Bearing in mind that it is only 350cc, do you think those sprockets would be suitable or should I try to source different ones?
Regards
John

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Re: Burman gearbox - AMC Clutch

Postby zwarts » Wed Aug 23, 2017 8:11 pm

John, I assumed that it was 500 as this is the G80 / M18 part of the forum! Std. gearing for the 350 is 5.8:1. If you use 18t engine sprocket with 16t gearbox and the AMC clutch you will be under geared at 6.1:1 top gear ratio. To get back to 5.8:1 you need a 17t gearbox sprocket. Alternatively, if you have a lively 350 engine, you could try an 18t gearbox sprocket with the std 18t engine sprocket giving 5.4:1 top gear and more relaxed cruising.

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Re: Burman gearbox - AMC Clutch

Postby 1957M30 » Wed Aug 23, 2017 9:11 pm

This maybe of help to you as on stripping my clutch out I found that
I had the Burman Box to AMC conversion.


http://www.jampot.com/forum/posting.php?mode=quote&f=11&p=176593

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Karol
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