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AMC gearbox

Classic Norton Commando Motorcycles.

AMC gearbox

Postby kebra » Wed Jul 01, 2009 3:48 am

Is there anybody here who could explain me the differences between AMC gearbox, and commandos gearboxes?
Is it possible to mount them directly on a 71 Commando frame's plates and engine?

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Re: AMC gearbox

Postby BrianK » Wed Jul 01, 2009 5:29 am

Mick Hemmings' video on Norton gearboxes explains the differences between the Commando (which I thought WAS referred to as the AMC gearbox) and the earlier Norton "laydown" boxes in detail.

As I recall, the earlier boxes don't have their mounting lugs aligned the same way the Commando one does, so mounting would not be a straightforward bolt-on.

Others will know more I'm sure.

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Re: AMC gearbox

Postby tpeever » Wed Jul 01, 2009 5:44 am

I believe the AMC gearbox was introduced about 1957 and used on all the AMC motorcycles (Norton, Matchless and AJS). The gearboxes on my 1961 Matchless G12, 1961 Norton Dominator 88 and 74 Commando are identical except possibly for gear ratios. Most of the parts are fully interchangeable.
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Re: AMC gearbox

Postby Jeandr » Wed Jul 01, 2009 6:12 am

I was told the biggest difference is the Commando gearboxes have the top mounting lug a bit smaller and there is a washer making up the difference to allow the gearbox to be removed easier from the cradle. I think the very latest gearbox shells are also thicker.

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Re: AMC gearbox

Postby bigstu » Wed Jul 01, 2009 6:49 am

Forget the Laydown box, that was a different beast, but with many similar parts.
The Commando gearbox is The AMC gearbox, but with subtle differences...of which I am unsure. I do know when they used the AMC gearbox for the Atlas they had a slightly different form on the camplate which the clutch pushrod ball operates on than the Commando.
So if you want to improve the 'action' of your clutch lever, changing to the different Atlas cam will make your clutch slightly easier to pull.

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Re: AMC gearbox

Postby dave M » Wed Jul 01, 2009 8:11 am

There are differences between Commando boxes and some of the earlier models, although they are all AMC boxes. As previously pointed out the top mounting angle of the casing on a Commando is different from pre-commando models, also the main-shaft is different to accommodate a totally different clutch. I believe that the gear ratios also vary between models.

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Re: AMC gearbox

Postby 79x100 » Wed Jul 01, 2009 10:00 am

Although referred to as the AMC box because it was brought in by them, it was based on the earlier Norton upright and laydown boxes rather than the Burman box that AJS/Matchless had used prior to that. The Norton box was in turn based on the earlier Sturmey Archer box. When they ceased production, Norton's had it made for them with a few modifications (by Burman !)

A shell numbered with an 'N' prefix is from a Norton and can be used in the Commando. Contemporary AJS / Matchless boxes (stamped with an 'M' I believe) will not fit as the mounting lugs are rotated roughly 45°. If you look at a picture of an Ajay/Matchy, you'll see that the gear change shaft sits low, roughly in line with the kickstart shaft and on Nortons the gear change is much higher.

Obviously, earlier comments about the internals apply and if I was doing it, I'd want to use all the upgrades but the basic shell is no problem.
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Re: AMC gearbox

Postby chris plant » Wed Jul 01, 2009 11:51 am

hi kebra,i beleive the pre commando amc box had the number and N prefix stamped on the main shell at the right hand side just behind the clutch cable entry,the commando box had the number stamped on the top mounting lug,the top mounting lug on the commando had aprox 1/4 ins machined off the left hand side(clutch side) for ease of installation(removal) from the cradle, as mentioned earlier subtle differences in gear ratios and mainshaft length apply,,matchless boxes were prefixed M and different mounting lugs, are you trying to identify a box you already have
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Re: AMC gearbox

Postby kebra » Fri Jul 03, 2009 3:45 am

thanks everybody for yor answers

have a good ride this week-end
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Re: AMC gearbox

Postby possm » Sat Jul 04, 2009 5:20 pm

Hi Every body
THe commando mainshaft is longer with a circlip grove.
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Re: AMC gearbox

Postby L.A.B. » Sat Jul 04, 2009 11:49 pm

kebra wrote:Engine:20M35152671

"20M35" should be 20M3S
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