G12 crankshafts

cf160 · Post by **cf160** » Fri Feb 24, 2012 6:52 pm

My two mod31 cranks (could be model 30 or 20 ,I suppose) are in decent shape ,but were retired for some reason or other from a race track on some beach in California over 55 years ago. One has a drive side splined shaft nearly three inches long.
The other's drive side splined shaft is only two inches long. There is no key slot on either one. Just eyeballing it, makes me think there is room for the main roller bearing, the sprocket and perhaps an inch for the rotor which may not be enough, it now occurs to me.
There is a small (1/4") threaded hole in the shorter shaft while the longer one has a 1/2" threaded hole in it. These must be for the flapper valves to screw into, I suppose
Dave(cf160)Edited by - cf160 on 24 Feb 2012 6:54:39 PM

wilko · Post by **wilko** » Fri Feb 24, 2012 7:13 pm

Just show pictures and the experts will tell.

Don Madden · Post by **Don Madden** » Fri Feb 24, 2012 7:51 pm

The one with 3in splines sounds like one for the cushion drive discontinued after 1956. The first factory 650cc were for the 1958 G12 & varients for export. Take the pics & measure stroke to be sure. Cheers, Don.

cf160 · Post by **cf160** » Fri Feb 24, 2012 8:43 pm

https://picasaweb.google.com/1009769639 ... tGo7ZrP9gE
This is the url of the uploaded images of the cranks
cf160

zwarts · Post by **zwarts** » Fri Feb 24, 2012 9:53 pm

The crank with the longer splines is a pre-57 type which takes a cush drive. It will be a 500cc or 600cc crank (the 650cc engines came with the AMC gearbox and therefore never had a cush drive on the end). The other is a post '57 dynamo type. You will need to carefully measure the stroke to determine if it is a 500cc ( same stroke as 600cc) or a 650cc item.
Bob.

cf160 · Post by **cf160** » Fri Feb 24, 2012 10:26 pm

Thanks for pointing that out , Bob. I probably would never have thought of that! I assume the 650 has a longer stroke ...
Dave

zwarts · Post by **zwarts** » Sat Feb 25, 2012 9:28 am

Yes Dave, the 500/600 stroke is 72mm and the 650 stroke is 79.35mm.
Bob.

cf160 · Post by **cf160** » Sat Feb 25, 2012 5:17 pm

Thanks so much for that , Robert. I know I will have a good time measuring finding the centres of the crank pins ...
Dave(cf160)
P.S. Bob: Have you written any service manuals for AJ's?
I would love to read them ...Edited by - cf160 on 25 Feb 2012 5:20:44 PM

dave16mct · Post by **dave16mct** » Sat Feb 25, 2012 5:59 pm

Dave, a quick way to tell is (from memory) 500/600 cranks are approximately 1 1/4" from the crankpin to the edge of the flywheels. The 650 crank is 1 1/8" approx. Dave.

cf160 · Post by **cf160** » Sat Feb 25, 2012 6:12 pm

Thanks Dave , That is a good indicator and one which I will be able to measure easily,
Dave(cf160)