G12 Crank

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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G12 Crank

Postby Chessiegolf » Sat Apr 04, 2020 8:34 am

Can anyone help please, I have a 1960 standard G12, the engine number is 60/G12 X3224. On the assumption that the crank has not been changed, does the engine number tell whether the crank is cast iron or nodular iron.

I've read conflicting accounts on the interthingy about when the changes came in but thought the owners club would be able to provide a definitive answer - the engine is built up so not possible to check for the ringing sound that a nodular iron crank supposedly possesses.

Thanks

John

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Re: G12 Crank

Postby JEAN-NOEL » Sat Apr 04, 2020 9:09 am

Hi John,
I have'nt found the answer...
Ernie Merriweather indicates the crankshaft has been improved, but without indicating a date or an engine number.
Roy Bacon wrote "From about 1960 its material was uprated for a high grade alloy iron casting to nodular iron..." No more date or engine number.

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Re: G12 Crank

Postby Chessiegolf » Sat Apr 04, 2020 2:38 pm

Hello Jean-Noel,

Thankyou for attempting to find the answer. It seems my bike is just at the transition point but it would be nice to find out, more peace of mind than anything else.

Quite a few people say that the extent of the crank snapping problem was exaggerated and I've no intention of overly stressing the engine, still I would feel happier if I knew the crank was the 'improved' version.

John

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Re: G12 Crank

Postby JEAN-NOEL » Sat Apr 04, 2020 6:02 pm

Hello John,
Do you have a dating letter ? This could give you a good indication : 1961 means October 60 to september 61. A G12CSR was road tested in France in July 61 @ 112 MPH, and was tested on 2700 km. The same bike (with a racing kit) won the the French championship 750 cc sports 62 et 63. In 64, it was an Atlas... This bike had presumably a nodular crankshaft !

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Re: G12 Crank

Postby Chessiegolf » Sun Apr 05, 2020 4:43 pm

I have a Certificate of Authenticity, issued by the Owners Club, so assume this is the same thing as a dating letter.
The certificate states that the bike was completed and tested on 28th March 1960.
The power is stated as 43bhp @ 5800rpm - that sounds optimistically high to me.

Regards

John

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Re: G12 Crank

Postby Mollbhan » Sun Apr 05, 2020 8:45 pm

Hi John, for what it's worth, my 61 G12L engine number X7213 has a nodular iron crank (I think, how can you be sure), unless you are planning on thrashing the engine I don't think it would matter. The Triumph Bonneville 650 produced 43bhp I always understood the humble G12 in standard form produced 35. Quite honestly I rather like the tractor like qualities of the big Matchless.

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Re: G12 Crank

Postby cbranni » Sun Apr 05, 2020 9:52 pm

Did I not read that nodular cranks are stamped with the letter N, my 61 G12 engine No X5634 had no such stamping, well I couldn't find it when I looked.

Colin
only dead fish go with the flow

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Re: G12 Crank

Postby Mollbhan » Sun Apr 05, 2020 10:58 pm

According to Ernie Merryweather in The British Bike magazine of the 1960s tapping the crank for the ringing sound is the easiest way to tell them apart, some nodular cranks were stamped with an N and some were not.

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Re: G12 Crank

Postby Groily » Mon Apr 06, 2020 5:07 am

cbranni wrote:Did I not read that nodular cranks are stamped with the letter N, my 61 G12 engine No X5634 had no such stamping, well I couldn't find it when I looked.

Colin


FWIW my '61 Mod 31 crank - or the one in the blown engine that came with the bike - was also a standard meehanite one. No N on it, and a very dull 'crump' when tapped with a hammer.

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Re: G12 Crank

Postby Chessiegolf » Mon Apr 06, 2020 6:19 am

Thanks for the replies.

The engine is installed in the bike and running, well it was before we all had to stop, so I'm unable to tap the crank and listen for the ringing/thud sound.

As a long past-it rebellious youth, I now have a lot more respect for vintage machinery and its uncomfortable to even maintain 70mph, let alone find out just how fast the bike will go, so I've no intention of willingly over-stressing the engine, however I would have preferred knowing what crank the engine was originally built with.

It just seems logical to me that AMC stamped the engines with a number so that they knew the mod state at the time of manufacture and that they would have maintained records. Perhaps the records disappeared with the demise of the company, but even so I thought the owners club would be the best source of knowledge.

Thanks for the many replies anyway.

John

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