G80 CP Burman mainshaft problem

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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G80 CP Burman mainshaft problem

Postby hatrack » Fri Nov 17, 2017 4:23 pm

I got the '47 G80 fired up today after its rebuild, despite our garage having been demolished and not yet rebuilt.
A crisp and easy start up and tickover settled down nicely, I thought I'd check gear selection before going out for a short trial run.
But then, "Where has the cluch action gone?"
An unbelievable amont of slack has suddenly appeared in the operating mechanism and I was imagining that the clutch actuating lever's pivot pin could be broken.
Particularly as its a grease lubed gearbox, I was a little shy of popping the outer cover off the gearbox to see, with no roof over its head, no work bench and only limited tools as most have been stored away until the new garage is up. But it was a nice sunny day so "au plein air" with an ex army catering tray to catch the drips, I got cracking.
Amid the dollops of semi fluid grease I was surprised to find the operating lever and its pivot all in order. But then amongst the grease, behind the kickstart quadrant, "Whats that?"
The clutch operating plunger and ball bearing, the kick start ratchet, retaining collar and nut lying where they should not be.
Once the gease was mopped up I tried refitting them. This went well, excepth that only two and a half threads of the mainshaft end protruded and the nut did not easily thread onto it.
Over a cup of tea to consider, I conclude that some previous owner sheared the end of the shaft, leaving just a few threads and put the nut back, hoping it would do. The surprizing thing is that it has been ridden quite a bit in this state, and held together just long enough for me to start it today.
If anybody knows better and can explain that its Ok to have so few threads for the nut, I'll be glad to hear.
I do have spare, later oil filled box, but that is buried in a box deep among other boxes in a confined space until the new, super dooper garage is available, and until then I have nowhere to check that box and make sure it has no secret horrors of its own.

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Re: G80 CP Burman mainshaft problem

Postby SPRIDDLER » Fri Nov 17, 2017 4:48 pm

Not something I've experienced but I can't see how that could happen (to the k/s assembly) and surely the shaft won't/can't have sheared off? Does the end look roughly finished? I can't think how it can happen, but maybe the mainshaft (did you mean the mainshaft?) isn't 'short' but has moved 'inwards' somehow. If you fit the nut and pull on it with pliers or Mole grips whilst turning it perhaps it will protrude a bit further................
Is the gearbox final drive sprocket and the drive chain run still aligned with the rear wheel sprocket?
Curious.

Exploded diagram of the CP box here:

http://archives.jampot.dk/technical/Tra ... d_View.pdf
Last edited by SPRIDDLER on Fri Nov 17, 2017 5:11 pm, edited 1 time in total.
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Re: G80 CP Burman mainshaft problem

Postby hatrack » Fri Nov 17, 2017 4:54 pm

The shaft does not look definitely "sheared off" but there was a ragged sliver of thread hanging off it.
The nut is almost twice the thickness of the available thread.
Putting the nut on and tightening would achieve nothing, as the nut only clamps the collar in place which fits on splines and retains the klck start ratchet which is free to slide and engae/disengage.

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Re: G80 CP Burman mainshaft problem

Postby SPRIDDLER » Fri Nov 17, 2017 5:17 pm

Not really sure how this can have occurred although the bits (plunger, ratchet, collar, nut) must have dropped down when you removed the cover (which I accept doesn't explain the 'short' shaft) but I've never had to delve inside a CP 'box so I'll defer to others who have.
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Re: G80 CP Burman mainshaft problem

Postby hatrack » Fri Nov 24, 2017 2:00 pm

Well, still without a garage or workbench, though the concrete base for the new "perfect motorcycle garage" was laid yesterday, The sun shone this afternoon and as I had had to jetwash the mud from garage base excavations off the drive, the tarmac was drying out nicely in the sunshine, so wheeling the bike from the garden where it is under a cover and chained to an apple tree, I got the cover off the gearbox and took another look.
There is not a cat in hell's chance of getting this nut back onto the mainshaft.
With kickstart ratchets removed, there is only 1/8" of threaded shaft. The nut which goes on is 1/4" thick, so there is something amiss there to start with.
But when the kickstart ratchets are in place, the outer ratchet which fits on splines on the mainshaft cannot slide far enough on for even that short threaded section to protrude. And so the nut cannot even begin to start on its thread.
It dosn't look as if the end of the shaft is broken, but the threaded section has only a very thin wall thickness, if a portion of the shaft had broken off it would be there within the casing somewhere.
As I said, I have a spare gearbox, but it is buried in a box, somewhere in a stack of boxes in the garden shed, untill the garage is ready.
Could somebody who is familiar with the Burman CP gearbox and has bits to hand, please tell me how long that threaded section should be.
Possibly the nut I have is not correct, but none of the usual AMC spares sources seem to list one. However, Draganfly tell me the will be making some, this December.
I am desperate to get this problem sorted by Christmas, as I have an entry for the Moto Giro d'Italia next April and need to put in plenty of miles before then to ensure it is ready for such a serious trip.

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Re: G80 CP Burman mainshaft problem

Postby Pharisee » Fri Nov 24, 2017 4:52 pm

A thought that may be silly... Have you had a look at the other side of the bike? When the nut 'fell off', has the whole clutch assembly moved outboard and taken the mainshaft with it? If it has, can it be persuaded to go back?

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Re: G80 CP Burman mainshaft problem

Postby hatrack » Fri Nov 24, 2017 6:12 pm

Its not that sort of problem. Its not a case of the mainshaft having moved. It is just a very short threaded section of the mainshaft.
Not something caused by the movement you suggest.
But thanks, I would love somebody to come up with a real, blindingly obvious solution that is the real answer.

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Re: G80 CP Burman mainshaft problem

Postby SPRIDDLER » Fri Nov 24, 2017 6:58 pm

Several techy articles on the CP 'box in Christian's Archive with sketches showing stripping/reassembly and the k/start assembly which may throw some light on the issue and provide an Eureka! moment.

http://archives.jampot.dk/technical/Tra ... r_1961.pdf

and another (better diagrams) here:
http://archives.jampot.dk/technical/Tra ... b_1941.pdf

and another:
http://archives.jampot.dk/technical/Tra ... motorcycle).pdf

It's probably a red herring but whilst browsing the archive I came across this in the 'Transmission' section:

Burman mainshaft length.JPG
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What if the Hokey-Cokey is what it's all about?

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Re: G80 CP Burman mainshaft problem

Postby hatrack » Fri Nov 24, 2017 8:17 pm

Thanks. Those pictures appear to show a longer threaded section than mine has, and the thread extending the full thickness of the nut.
Sadly no Eureka moment, but confirming my suspicion that something is not as it should be.

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Re: G80 CP Burman mainshaft problem

Postby Chris Berry » Fri Nov 24, 2017 9:34 pm

I find your problem very interesting. When I purchased my 1948 G80 around 18 months ago I also bought another gearbox to go with it. After overhaul I replaced the old one with the new. when I pulled the side casing off the old gearbox I was astounded to find that the nut retaining the kick start assy onto the mainshaft had been welded on. Not just a little tack but a full peripheral weld. I have no explanation as to why someone would do this but, by the time i ground the weld off, the main shaft was next to useless. I noticed that on mine also there was not much thread on the shaft to purchase on so may be this was the reason for the weld

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