GB Gearbox

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Dave.Holloway
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GB Gearbox

Postby Dave.Holloway » Mon Aug 12, 2019 4:15 pm

New to this forum having found myself an 58 Huntmaster. Usual in depth check has revealed a knackered thread on the mainshaft, clutch side. I am given to understand that i can (if i am lucky or very lucky ) remove the gearbox outercover , undo the nut on the end of the main shaft and carefully withdraw the shaft replacing it with a suitable rod, then slide a new shaft in. I am pretty nifty with a lathe so maybe able to reclaim the thread and make a "fitted" nut. Am I correct in this assumption about the mainshaft or is it going to end in tears.
(I have to say the chaincases are a bit "weedy" compared to BSA ones)

Simon.Gardiner
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Re: GB Gearbox

Postby Simon.Gardiner » Mon Aug 12, 2019 4:45 pm

Hi Dave, and welcome!
I've not heard of that trick being used with a GB mainshaft before but no doubt someone will be along with more knowledge on that.
If that doesn't work it is possible to strip/rebuild the box without removing it from the bike first.
You might want to look at this thread with good info on reclaiming threads.
viewtopic.php?f=4&t=8831

HTH

SG

(PS be very careful about criticising Mr Ariel's fine chaincases around here.....!! )
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)

Dave.Holloway
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Re: GB Gearbox

Postby Dave.Holloway » Mon Aug 12, 2019 9:13 pm

looks like i will be making a thread rolling tool. The end couple of threads have been dot punched by the look of it.

Mick D
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Re: GB Gearbox

Postby Mick D » Tue Aug 13, 2019 8:26 am

Dave.Holloway wrote: I am given to understand that i can (if i am lucky or very lucky ) remove the gearbox outercover , undo the nut on the end of the main shaft and carefully withdraw the shaft replacing it with a suitable rod


Hi

This is not possible unless you take off the inner cover which houses the kick start side main shaft bearing - the main shaft splines will not pass through the bearing inner.

Why not take the opportunity to completely strip the gearbox and give yourself some peace of mind about it's condition? It is a GB44 isn't it?

Regards Mick

Dave.Holloway
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Re: GB Gearbox

Postby Dave.Holloway » Fri Aug 16, 2019 7:12 pm

I have managed to reclaim the GB gearbox (GB6H52) mainshaft. Someone had liberally dot punched the end threads , The nut was totally U/S . A new nut from drags now fits and is a good tight fit. Rebuilt the gearbox this afternoon and it selects all gears. First time i have done one of these and quite simple really. Next Question . : When the clutch sleeve is slid onto the mainshaft it then comes up against the ends of the big splines. There is a gap between it and the sprocket nut , about 0.200. Does this space need a loose spacer of some sort? It is a space which will get road muck as there is no front FERC part. I was thinking of a bit of rubber tube in fact.

Mick D
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Re: GB Gearbox

Postby Mick D » Fri Aug 16, 2019 7:42 pm

Hi

If the GB6 main shaft is correct for the gearbox it won't fit a 58 FH, which has a FERC and therefore an extended main shaft, the correct gearbox is a GB44 - see here for more info:

viewtopic.php?f=4&t=8843

Regards Mick


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