crank balance

Hello everyone I have a question. I have a 1970 Norton Commando that im building. has stock rods that are in good conditon. pulled the crank apart and cleaned the trap. measured and polished the journals. It needs new pistons going from .020 to .040 oversize. How criticle is is balancing the crank or can I take a chance and just put it together. thanks.

Its less critical than any rigidly mounted engine but as you have it out then get it statically and dynamically balanced. They were statically balanced, ie the up and down bit, at the factory after a fashion but never dynamically balanced which is the side to side balance. Beware of Balancing shops that have balanced Harley cranks saying they can do Brit cranks, they have a reputation for messing it up. Go by a recommendation from this forum.

I just rebuilt mine and did not even consider to balance the crank. It runs fine now.
I don't think I could tell the difference even if there was a balance problem and in addition I
don't get into the high rev zone very much.

But I had a local shop measure the con rod big ends and they found that while the crank bearing surface just needed a polish, the con rod big ends were slightly ovaled (along their length axis due to the pull down motion). I forget how much but perhaps 1 1/2 to 2 thou.
So they resized them and the big ends now have 1 thou clearance as per spec.

Ok thanks for the feedback. will check for ovality. probably will just put it together. I dont trust sendng it out will be safe with all the fuge factors.

A well balanced crank will give you a smoother ride all up, your motor will not wear out and get a lot more mile out of the motor, also helps with over sized pistons and will rev a lot harder, but if its just a street bike without doing high end revving, just putting the crank together will be fine for normal running.
My 850 crank is balanced at 72% balance factor but I also run a 2S cam, 40 oversize Hepilite pistons with min clearance, head has been shaved and ported, run open exhaust system and is hard mounted in the Featherbed frame, it runs very smoothly for a hard mount motor and the motor revs freely, just got to watch how far I do rev it, but the balanced crank plays a big part in its high performance.

Ashley

Ok got it. sounds like a cool bike. Ill look a some previous threads on this topic

Peter Restaino said:

Ok got it. sounds like a cool bike. Ill look a some previous threads on this topic

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You've got it out - now's the chance. I would definitely have it balanced.
Remember there is static balance and dynamic balance.
A crank that has been balanced statically may look to be in balance but will likely be trying it's damnest to screw it's way out of the cases when running. Dynamic balance corrects this left-right imbalance.
I would recommend you choose someone who can do both.
...and, at the risk of opening an "what oil? thread", be able to tell them what balance factor you want.
Cheers