GB Gear Selection

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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GB Gear Selection

Postby mickm » Tue Sep 03, 2019 8:31 pm

My gearbox is becoming difficult to change up,mainly from second to third. Sometimes forth is not easy. I find that two attempts at the change and it will go in. First attempt it just feels dead then release the leaver on second try and it will shift. This is not repeated on the downward shift. The clutch is new that's hub,centre,and plates. It clears well and doesn't drag. I've stripped the box the bearings will be replaced, nothing else looks bad. Any ideas ? Mick

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Re: GB Gear Selection

Postby Mick D » Wed Sep 04, 2019 6:28 am

Hi

I assume you have an AMC gearbox.

Your problem probably stems from a combination of several worn components so clean everything and check:

End float of the main and layshafts, (remember to take into account the thickness of any gasket you use).
Radial play in the gear bushes, particularly pn 040048, bush for mainshaft second gear.
The gear drive dogs for burrs.
The main and lay shaft splines for damage.
Wear or damage on the gear ratchet assembly.
It's unlikely that the cam plate is the cause but have a look anyhow.

Have you done anything to the gearbox that could have initiated the problem or has it just gotten worse with use?

Regards Mick

Edited to correct my assumption to AMC, prompted by John's post below - Thinking AMC, typed Buman :oops:
Last edited by Mick D on Wed Sep 04, 2019 8:06 am, edited 1 time in total.

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Re: GB Gear Selection

Postby Pharisee » Wed Sep 04, 2019 7:35 am

I've just stripped and re-built a Burman GB gearbox. The only place I found any appreciable wear was on the two dowels that are fitted into the ends of the selector rods and locate in the groove in the selector drum (the GB gearbox doesn't have a 'cam plate' as such). I hadn't noticed any particular difficulty in selecting gears but wear on the dowels might be a contributing factor.

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Re: GB Gear Selection

Postby mickm » Wed Sep 04, 2019 11:07 am

Thanks Pharisee that's the sort of thing I'm looking for. My gear indicator on the outer cover has been cut off by the P.O so I'll fit a new cam drum and pins,along with bearings and seal. Thanks again, Mick

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Re: GB Gear Selection

Postby Pharisee » Wed Sep 04, 2019 12:40 pm

mickm wrote:Thanks Pharisee that's the sort of thing I'm looking for. My gear indicator on the outer cover has been cut off by the P.O so I'll fit a new cam drum and pins,along with bearings and seal. Thanks again, Mick


At the risk of being drummed out of the owners club, I get most of my Burman gearbox parts from "Draganfly Motorcycles" : https://www.draganfly.co.uk/burman-gearboxs/gb-type
They keep all the parts but you have to use their numbering system (which may be the original Burman part numbers?). The part numbers in the AJS /Matchless parts lists are for the most part different and not understood.

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Re: GB Gear Selection

Postby mickm » Wed Sep 04, 2019 2:18 pm

I've bee the phone to them already, yes they are good. As I have a pall In the Ariel OC,they are also very helpful.

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Re: GB Gear Selection

Postby Mick D » Wed Sep 04, 2019 3:45 pm

Hi

I have just rebuilt a GB44 box and in my experience AMC Classic Spares are my 'go to' supplier, if they stock the item they usually undercut Draganfly by some degree.

Strange that your bike has a Burman GB gearbox or were you not referring to the 57 model in your profile.

With the exception of the cam plate comment, my advice above reads across to a Burman, (GB26?), gearbox in which case the bush to check is the second gear layshaft bush pn 3650-52, (in the Draganfly Catalogue).

Regards Mick

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Re: GB Gear Selection

Postby mickm » Wed Sep 18, 2019 11:14 pm

Hi Mick my bike is a bit of abitsa the gear box is a burman. As i live north Essex I visited Dragonfly with a box of bits. They were very helpful and pointed out bits that had been modded by P/O and perhaps a hammer or two. I replaced the selector drum, fork & return spring. It works ok on the bench, but will not attempt to try it on the road as the dynamo has given up. So when that's sorted I'll give it a run. Thanks for your time and help. Mick M.

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