31 crank cases compared with Mod. 30 cases

cf160 · Post by **cf160** » Thu Jul 19, 2012 11:03 pm

I wonder if some knowledgeable souls know if the cases for the 1956 and '57 Model 30's were identical to the Model 31 cases of the same years? I have just found a good '57 drive side case and wonder if it
will bolt up correctly with the Model 31 timing side I have in the basement? I know the strokes are slightly different, but that may not matter.
Dave(cf160)

Groily · Post by **Groily** » Thu Jul 19, 2012 11:53 pm

No 650s till '59 Dave, and the first 650 year was a one-off as regards the top end at least. Whether the cylinder stud spacing and other details of the cases for 650s were exactly the same as M30s / G11s from '56 and '57 I don't know for certain. All sorts of little things to think about, including the siting of the oil pressure relief valve, and other oiling and oil filter assembly mods (of which there were a few between '59 and the end of 650 production). Someone here will know for sure. Whenever I think I have something figured out, a fresh question leaps out and smacks me in the chops.
What I have learnt, the hard way, is that any attempt to mate unmatching c/case halves requires very careful work, and possibly some precision machining. Athough the factory did a pretty good job of things, you'd be optimistic to assume no minor rectification was required. Even if only to flatten the cylinder barrel base surfaces.
The controls required extend to the camshaft bushes as well as the main bearing housings. You'd want to replace and line ream the cam bushes afresh in a mix and match set. (Or I think so/would do it anyway.)
I always start here ('Twin Changes') when trying to mix and match anything: https://www.jampot.com/article_read.asp?id=20
It's a great survey of the main differences between years.
Hope I'm not being a pessimist but the stakes could be expensive in terms of short engine life, if things aren't spot on. None of these engines is cheap to mess with any more unfortunately!

cf160 · Post by **cf160** » Fri Jul 20, 2012 12:27 am

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)

wilko · Post by **wilko** » Fri Jul 20, 2012 1:00 am

The 600's are a sweeter engine anyway. Square bore and stroke.

cf160 · Post by **cf160** » Fri Jul 20, 2012 3:56 am

Have you driven both, Doug? Is there much difference in performance?

JEAN-NOEL · Post by **JEAN-NOEL** » Fri Jul 20, 2012 10:11 am

Hi Dave,
I had in 1965 an AJS M 30. It was a nice, but leaky machine...At that time there wasn't speed limits in France (only 60 kph in town) and it was often driven full throttle...At 55 MPH it was running as a seewing machine, and vibrations were really unpleasant at 140/150 kph. Moto Revue tested it in 1957 at 156 Kph/97.5 MPH, solo with leather clothes, and 139 KPH/87 MPH with a pillion, and touristic clothes. Exactly the perfs tested by Moto Revue for the Norton 99. It was reliable : always got home by itself ! I sold it in 1967, and the next owner broke the crankshaft in two parts ! My G12 CSR is a 1965 model, bought in 1993. I never exceeded 90 kph 55 mph / 2900 rpm so the engine is "hectic": the vibrations are very unpleasant, and I dare not rev it! I'm really afraid of breaking something! So I decided to overhaul the engine with a new crankshaft and new connecting rods cnc machined. The G12 CSR in 1961 was a very fast machine, tested by Moto-Revue @ 180 Kph /112.5 Mph ! I translated the road test in JAMPOT No. 588 of July 2001. I think for a tourist use the M30/G11 is more enjoyable, even if it has less torque.

cf160 · Post by **cf160** » Fri Jul 20, 2012 5:06 pm

Merci bien pour ca ,Jean-Noel. It sounds like the extra 50 cc and 5mm longer stroke doesn't make
a lot of difference. The model 30 must be almost
the same as the model 31.I never noticed vibration
problems with my 31 although there was a little, not as bad as the Nortons which are manageable.

JEAN-NOEL · Post by **JEAN-NOEL** » Fri Jul 20, 2012 6:41 pm

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 éach !) 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.

cf160 · Post by **cf160** » Fri Jul 20, 2012 7:13 pm

Hi Jean-Noel:
What year is your Model 30 ? I think you said it was 65 , but I am not certain as it might have been quite a bit older. My model 30 will be using a
1956 or '57 engine and a '56 frame . I have a few sets of older pistons and barrels. I may be able to send you a set as I will not need all of them .
If your cylinders are .040" oversize already,
These pistons may be too small as they are
mostly standard, but there are a set of Grandini
which are 73mm but they have been smeared a little on the skirts by a guy who put them in a standard bore (72mm +.020) Don't ask me how he did it! I bought them for 20 pounds in hopes of using them in my model 30 , but I am not sure
about the matter...

JEAN-NOEL · Post by **JEAN-NOEL** » Fri Jul 20, 2012 8:03 pm

Hi Dave,
I bought my M 30 in 1965, but it was a 1957, very fine vith its "torpedo pilot lights" and blue strip on the centre of the rims. I sold it in 1967. Now I have a 1965 G12CSR. I am waiting to restore it a brand new and improved crankshaft and a pair of new conrods from Thunder Engineering. GANDINI pistons are suitable for a touristic use. Their weight, circa 380 grams give a balance factor of roughly 63 %. Thank you for offer.