Gearbox shell differences

Information relating to the Matchless G15 or AJS Model 33 750cc twin. This also includes the G15 Mk II and the G15/45
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Gearbox shell differences

Postby mdt-son » Sat Oct 06, 2012 1:42 pm

This week I was made aware of a comment by an author (may have been Roy Bacon) that the gearbox shell was different for the G15/P11 range of bikes.
Going by the part books, the std. shell has p/n 040144 while the hybrids had p/n 040413. The 500CS had yet another shell, p/n 040443.

Can anyone shed a light on the differences? Material? Heat treatment? Different machining? Bolt sizes for fixation to the engine plates?

For the roadsters, the hybrids and the 500CS, upper fixing bolt size is identical at 7/16". The pivot bolt size was initially 9/16" (p/n 000318) until MY1963 and for MY 1964 on it appears to be of smaller size (7/16") for the hybrids and all roadsters (p/n 024276, 033153).The 500CS stayed with the 9/16" bolt it seems.

I am beginning to think that the std. shell was actually 040413 for MY1964 on, but maybe there was another subtle change?

Any information on these shells is most welcome.

Knut Sonsteby

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Gearbox shell differences

Postby Don Madden » Sat Oct 06, 2012 5:19 pm

For the AMC gearbox, touring bike 650cc had a single primary chain adjuster between the plates with the adjusting bolt head bearing on the top bolt. The case had a scallop cut to expose the bolt. Scramblers had two adjusters, eye bolts mounted outside the rear plates & no notch for an inboard adjuster. This would account for some different part numbers. Compare the parts for the Hybrid adjusters with these. Note that Norton Commandos with the belt conversion also use the dual adjusters so the belt can be trued, critical in flat belt drives. Cheers, Don.

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Gearbox shell differences

Postby PGGuse » Sat Oct 06, 2012 5:44 pm

May this help
http://autos.groups.yahoo.com/group/Ajsmatchless/message/45368

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Gearbox shell differences

Postby mdt-son » Sun Oct 07, 2012 1:51 am

Hello Paul & Don,
Thank you for the link Paul, and Don for pointing out location of the adjuster(s). The scallop cut found in the roadster and hybrid shells plus the altered pivot bolt may explain the part number differences. However, the assertion made on weakened 'M'-type shells in the message referred to is unsubstantiated and I don't believe it. The design flaw with the AMC gearbox shells of both types (Matchless and Norton) is to be found at the drive side, where wall thickness and poor bearing bosses cause warping under load, ultimately resulting in a fatigue failure. I have seen this failure on half a dozen shells. Unfortunately the gear clusters and shafts suffer too. Even my 18S had a bent shaft which was not caused by an engine seizure.

Rgds, Knut

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