2a crank in mk3 cases

I'm about to start building my motor (mk3). I'm not going to fit electric start and was wondering if I should drive side shortened and rethreaded or fit a mk2a crank but I seem to recall there's a very slight difference in width, if so do I shim it either end at bearings or at flywheel. I would appreciate any feed back from anyone that has swapped cranks.

There's no need to shorten the Mk3 crank. All you need to do is keep parts 21, 22 and 23. If you shortened the crank you would then need to move the stator.

I've had one engine with MK3 cases and an earlier crankshaft. It was the ex-Martin Adams Commonwealth race bike. It had two aluminum shims, one in each side, between the crankcases and the bearings, to make up for the difference in crankshaft width. If I recall correctly, the shim on the timing side was thicker than the one on the drive side, maybe to make sure the oil seal was properly located on the crankshaft. It worked fine. In your case, it all depends on what sort of primary you plan to use. You haven't said if you plan to still use the MK3 primary components, in which case you need to keep the MK3 crank as L.A.B. said, or an earlier primary, in which case you need to machine the MK3 crankcase to match. If you plan to use a belt drive and custom cover, you can do it either way. But if you do that and still want to keep the alternator, you will have to fab up some sort of way to mount the stator.

Ken

Are you going to use a Mk3 primary case or an earlier one? If using the mk3 primary case there is no need to shorten the crank.

I should have mentioned in original post that it will be running open primary belt drive, I want to keep it as slim and tidy as possible.

roadrunner said:

I should have mentioned in original post that it will be running open primary belt drive, I want to keep it as slim and tidy as possible.

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In that case, you can do it either way, stock MK3 crank or earlier crank. The difference is that the MK3 cranks end will stick out a little wider. In either case, you will need to sort out a way to mount the stator. One option is to cut up a MK3 inner primary case to make a mounting for the alternator. That's what I did.

This is on a 1007 cc Maney engine with Maney cases and crankshaft, but the same technique would work with stock cases and cranks.



Ken