New Crankshaft

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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Re: New Crankshaft

Postby GSAX1 » Thu Nov 29, 2012 4:10 pm

There is no indication that the center main bearing causes more vibration. There was a comment in one magazine a long time ago, and this has been repeated over and over again. I have never seen a technical explanation to back up this claim. If anyone has one, please come forward with it. To me the design seems very sound, at least on the 500 and 600 models with their lower power.

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Re: New Crankshaft

Postby baggy » Thu Nov 29, 2012 7:16 pm

I love them twins & all of the bits inside , put together right , they go like the clappers , just ask john hattersley , none of you lads could catch him in 1960 or now . Regards Baggy

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Re: New Crankshaft

Postby wilko » Thu Nov 29, 2012 10:43 pm

I too, don't understand that blob of iron in the middle Dave. Someone explain?

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Re: New Crankshaft

Postby Stuoyb » Fri Nov 30, 2012 8:29 am

Not having been involved in the detail of this project I cannot comment exactly on this specific aspect.

I did however see many similar appendages on castings during my many years working in the motor industry. These were generally referred to as casting "bridges" or "braces".

Looking at what the crank is going to look like when finished, then the (relatively small) centre main is the obvious weak spot. Whilst the outside shell of the casting cools reasonably quickly it takes some considerable time for the core to be fully hardened. Without a "bridge" where the centre main is, the weight of the flywheels either side of the centre main could cause the the casting to come out of alignment. The extra material around the "weak spot" would prevent this happening.

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Re: New Crankshaft

Postby baggy » Fri Nov 30, 2012 4:41 pm

its where they pour the molten mix in lads , leaves a lump . Middle & out . ye ha !

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Re: New Crankshaft

Postby cf160 » Wed Dec 05, 2012 4:12 am

Thankyou for the explanation ,Stuoyb. That does make sense
Dave cf160,in Ottawa ,Canada
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Re: New Crankshaft

Postby cf160 » Sat Dec 08, 2012 4:16 am

Surely the strength and enduring nature of these twins is largely because of the centre main bearing giving the crank added
stability and freedom from minor disturbances like vibration that affects the other British Marks which have only two main
bearings , one being, as in Triumph and BSA, a mere bushing . My twin Mod. 31 had 61k miles on the clock when I bought it. I logged another 6000 before tearing it down for rebuild.
Long live the great design of the designer (whose name I forget)
cf160,Dave
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Re: New Crankshaft

Postby Don Madden » Sat Dec 08, 2012 8:09 pm

The "Other" makes of British twins, BSA, Norton, Triumph, used forged steel crankshafts with bolt on flywheels. The makers knew they would flex, (as all crankshafts do), so used ball bearings or the superblend barrel shaped rollers on one end to occomodate the flexing. The superblend is similar to the "spherical", bearing used on the early BMW bikes for the same reason. This bearing has barral shaped rollers, too. The AMC twin crankshaft before the so-called nodular, for economic reasons, was cast in Mehanite cast iron & too brittle to allow much flexing. They were fine as 500/600 but the 650 version were pushed to the limit. Many broke over here as they were fitted to the 1958 G12CS & used for desert racing. Frank Cooper, the importer for the Western US told me about the many failures but the factory answer was "They are not breaking over here".) Typical attitude for them, not realizing that more bikes were sold here than all the rest of the world together, & none could survive without the North American market. Cheers, Don, The Colonial.

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Re: New Crankshaft

Postby GSAX1 » Sat Dec 08, 2012 9:09 pm

There was a faulty batch of 650 cranks. Perhaps they all went to the US ;) ?

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Re: New Crankshaft

Postby Malleon » Sat Dec 08, 2012 11:47 pm

Perhaps these should have been dumped in the harbour (harbor?) at Boston...

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