31 crank cases compared with Mod. 30 cases

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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31 crank cases compared with Mod. 30 cases

Postby cf160 » Fri Jul 20, 2012 7:17 pm

Just weighed the Gandini piston with pin and one oil ring --- 384 Grams. This is a heavier piston than the others I have : BHB --311 Grams but std. size
The Heplex ones I have are lighter , but also Std. size.
David J.Potter

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Re: 31 crank cases compared with Mod. 30 cases

Postby mdt-son » Mon Sep 14, 2020 9:03 am

JEAN-NOEL wrote:Hi Dave,
to be honnest, my G12CSR is not a good sample : i have fitted JP pistons wich are a lot heavier (98 grams/3.5 ounces �ach !) than the original ALCAN...The recommanded factor balance, 65 %, decrease to around 50 % with these pistons, explaining the excessive vibrations. For your project a precise assembly with modern sealants, and the alloy chaincase of the 31 should remdy in large part the poor sealing I had noticed on my 30.


Hi Jean-Noel,

Is the balance factor you quoted above (65%) a factory specification, and if so, I am keen to know where it is documented. Furthermore, please state if the figure is measured dry or wet (the "wet" figure includes weight of oil in the oiling gallery).

I am really surprised by your reduction of BF for certain pistons. Why? The BF is a percentage of the reciprocating weight (usually 100% piston weight and 30% of the conrod weight, or 100% of the conrod small end weight as measured on a double knife set-up). I can't see a justification for reducing the BF for your choice of piston. If you did this, it may explain why your bike exhibits excessive vibrations above 100 kph. There may be other causes of course, like wheel eccentricities etc. This thread is an old one and maybe you have solved the vibration problem by now.

Best regards,
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Re: 31 crank cases compared with Mod. 30 cases

Postby JEAN-NOEL » Mon Sep 14, 2020 10:36 am

Hello Knut,
I have sent to you an email with a data sheet from, I think, Markus Graef. I don't know more about the balance factor used by the factory for the 650. Eventually I did rebuilt my engine with original crankshaft and conrods, + Gandini pistons from AMOC SPARES, and my deposit for a new crankshat was refund by the Club. If nobody has better informations, you could ask T & L Engineering what balance factor they recommend for the use you want to make of your bike.

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Re: 31 crank cases compared with Mod. 30 cases

Postby Groily » Mon Sep 14, 2020 11:01 am

When T&L did mine - prototype new crank, JPs at 384 gr excluding pins etc etc, the answer they delivered me was 61%. Which has been OK-smooth for me, for road use at not-slow but not mad bad speeds (I very seldom go beyond 110kph crusing in top, but do use the revs in 2nd and 3rd.) I'm geared (now) at 21-42-19-42, standard was - I think? - 23-42-16 (17CSR) -42.
Would post the data sheets, but too big I'm afraid. Anyone welcome to them - but by e-mail!

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Re: 31 crank cases compared with Mod. 30 cases

Postby ajscomboman » Mon Sep 14, 2020 11:29 am

As the crankshafts have been mentioned in this post I'll chime in and say, finally the dynamo buggers are all finished and have been checked over by Markus. I'm just awaiting the shipping details and costs and then once I have them here at Kettering I'll be in contact with all those who have paid deposits. It's been a rather painful process but it'll soon be over.

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Re: 31 crank cases compared with Mod. 30 cases

Postby mdt-son » Thu Sep 17, 2020 6:46 am

JEAN-NOEL wrote:Hello Knut,
I have sent to you an email with a data sheet from, I think, Markus Graef.


Not received and I sent you a PM. See Message Box in this forum.

-Knut

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Re: 31 crank cases compared with Mod. 30 cases

Postby mdt-son » Thu Sep 17, 2020 9:01 am

Jean-Noel kindly passed the Excel sheet to me. I am not sure how the originator arrived at 65%. Anyway, we need a database of piston weights and a consolidated approach to balancing.
I guess it's up to me to dig into this. I would like to research the possibility of greatly reducing reciprocating weights. In the Norton community, special piston assemblies are available weighing in at 280 grams or less.
AMC pistons always were very heavy and I wonder why the works did them like this. Worst example I have encountered was the Typhoon (600cc) piston.

Food for another thread.

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Re: 31 crank cases compared with Mod. 30 cases

Postby clive » Sun Sep 20, 2020 8:47 pm

cf160 wrote:Thanks ,Bill:
I was mistaken again as usual. I guess that means the engines I thought were 650's are both actually 600 model 30/G11's . Well , that will be
all right by me. My good 31 is somewhat more complete and in light of your advice ,Bill, my model 31 project becomes a model 30 project. I have gathered all the parts needed but for a few
things like oil pressure relief valves and such. Thanks for the link to the Twin History.
Dave(cf160)


Don't be too sure Dave. Just to confuse things in 58 they produced not only the G11 but later in the year an uprated 650 version the G12. It was according to the parts list in the archives http://archives.jampot.dk/book/Spares_lists/1958.Spares.List.G12CS.pdf only the barrel and pistons that were different. I think most were exported to the US.
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Re: 31 crank cases compared with Mod. 30 cases

Postby ajscomboman » Mon Sep 21, 2020 6:25 am

clive wrote:Don't be too sure Dave. Just to confuse things in 58 they produced not only the G11 but later in the year an uprated 650 version the G12. It was according to the parts list in the archives http://archives.jampot.dk/book/Spares_lists/1958.Spares.List.G12CS.pdf only the barrel and pistons that were different. I think most were exported to the US.


Now it is my understanding that the 58 650 was a stroked 600. The basic 600 barrels, cases and heads were retained however the new 650 crank was installed and using very short high pin pistons they were able to keep the 600 motor bits. However being short pistons they were prone to slap hence the change to the later 59 type and the barrel lengthened. Didn't Don Madden do a write up on the 58 650 motor highlighting all this? If it wasn't Don it was certainly one of our US cousins as I've read about the changes and seen the pistons first hand. You can't mistake them for anything else as they are very short compared to run of the mill types.

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Re: 31 crank cases compared with Mod. 30 cases

Postby matman » Wed Sep 23, 2020 9:10 pm

ajscomboman wrote:
clive wrote:Don't be too sure Dave. Just to confuse things in 58 they produced not only the G11 but later in the year an uprated 650 version the G12. It was according to the parts list in the archives http://archives.jampot.dk/book/Spares_lists/1958.Spares.List.G12CS.pdf only the barrel and pistons that were different. I think most were exported to the US.


Now it is my understanding that the 58 650 was a stroked 600. The basic 600 barrels, cases and heads were retained however the new 650 crank was installed and using very short high pin pistons they were able to keep the 600 motor bits.

My Model 30 engine received a long stroke crank rather unintentionally... The engineer who rebuilt my engine with a used crankshaft didn´t know about the different stroke lengths, so he showed himself rather surprised when the piston domes rose above the upper edge of the barrels. He solved that by machining aluminium plates and fitting them under the barrel´s bottom ends - which has since then lasted for 18 years and 20'k mls. or so. In the meantime I had to change pistons once and used original 600 ones - I had to shorten the skirts by half the stroke difference, which should be 3,25mm (79,3 vs. 72,8mm or 6,5mm difference in stroke).
Still a fine running engine. ;)

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