Single gearbox mainshaft lengths


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Hello Folks,
As I'm about to source a seperate gearbox for the device I am to build, it would help if I knew the distance from the centre of the gearbox sprocket to the back wall of the clutch basket for Burman BAP, and post 1957AMC gearboxes. I'm told that the GB 22 Burman boxes have a shorter mainshaft ( which I've been offered ) Any information for these boxes and anything else that might fit the bill would be of great help
Many Thanks, Phil Primmer


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Gearbox for 600 special


My advice would be too forget the Burman box for your hot special. It simply is not up to the task. I would definitely go for the Norton/AMC box. IIRC, you need the long-shaft variety (Commando?) with the serial number beginning with N. Not only is it a more robust box, you get the benefit of a really slick gearchange.

Peter B

Tom Gaynor

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Norton boxes

And Sid Biberman, who did exactly that on his 600 Comet, can tell you how. I have pics, I think, that Sid sent me. A bit more money spent, and a Newby belt drive and clutch can be fitted. No oil means no leaks...


Black Flash

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you should be looking for an AMC Norton box stamped N or NA.
they have the longer shaft.
I saw the pictures of the Biberman conversion and have built a Comet with this box myself. What I personally do not like on Sid's conversion is that the primary tension solely relies on locking the gearbox lugs between the plates.
on a hot plonking single this is bound to call for trouble I think.
On my bike I cut the lugs off the AMC box and replaced by machined ones the BAP way, which was a very extensive job ( you also have trouble with the valve lifer arragement doing what I did, which led to another modification).
I am sure there is an easier way to to it by using an adjusting bolt in AMC, Norton, BSA or Triumph fashion.
this will definately stop the gearbox from moving and thereby changing the primary tension.

If you need a hint call Bob Dunn.


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I have measured the gearbox that I have taken off the Comet on my workbench and the distance between the centre of the gearbox sprocket and the rear of the chainwheel is 1.208",,though the nearest imperial size is1 7/32"(1.21875")..hope this helps Phil...John