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Jan 22, 2008
Will 750 and 850 crankshaft interchange. Is it possible to repair a damaged key way on an 850 crankshaft?
Yes the 750 cheeks will bolt on the heaver 850 flywheel, If the 750 cheek was early 71 and before beware of a crank width change that took place.
To quote our moderator:
And what would the "R" stand for?

The Commando main bearings changed to the MRJ A30 (NJ/NF 306 [E]) C2 roller type around late 1971, as a result, the overall width between the bearings was reduced apparently, (the timing side main bearing journal diameter also increased according to Norton Service release number 68) and a new reduced width crankshaft, part 063106 was introduced to match the new bearings, these cranks having the 'R' stamp.

Quote from The October 1971 Norton Service Release No.68:


" The later crankshaft of reduced overall width is identified by the letter 'R' stamped onto the timing side crankshaft cheek adjacent to the bearing location. The designed crankshaft fitted end float for the new arrangement is 0.010in. to 0.024in. (0.254mm to 0.6096mm)."
Good stuff L.A.B.
Thanks Norbsa, and I believe the balance factors are different between 750 and 850 cranks? The 850 Mk3 electric start L/H crank mainshaft is also different (longer) than the others.
Yes the mark three strikes again ,Dave C had said it was unique but didn't say why. Buy the way, a key way is not that hard to weld up and re-machine and that is what I would do. But we have good welders and machinists here in Flint.
Hi there, the MK3 crank is 3mm wider between the faces of the bbearing and thus the MK3 cases are 3mm wider, you can fit a earlier crank into MK3cases but must shim behing inner races (1.5 mm thick each side).Don't ask why I know it, of course the late crank are different on the drive , as the shaft is longer, and the crank bolts are stronger, without mentionning the notch on the timing side cheek which could be used to align through the case, with the plug (for TDC or 28°?). The frog
Hey froggy,

Thanks for your post about the MK 3 crank peculiarities. The slot on the timing side of the crank is for indexing it 28 degrees BTDC.
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