commando crankshafts

Stephen Hill said:

Thanks SteveA. The only info have on the 850 crank is the owner's notes, so no idea what rods or pistons he used to balance it. It was used or intended for a Rickman race bike he had. I assumed that the balance for "solid mounting" in his words would would not work well in a stock iso suspension Norton. Perhaps incorrectly.

The only applications I am considering is in a stock bike ridden sanely, so don't see the need for a Molner flywheel.

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I run a Rickman and indeed a high balance factor (78%). But I also use JSM lightweight pistons and rods with less reciprocating weight. So the same crank with heavier pistons and rods would have a lower factor!

The PO is right to suggest a high balance factor wouldn't be 'correct' choice. But a lot of things that aren't correct work fine.

And some of the 'standard' piston options out there are pretty heavy! But they would be fine for the use you suggest.

If you don't feel your plans need the Molnar flywheel, you probably don't need to rebalance the crank!

The very first thing I did when building my 850 motor was rebalance the crank. I was about to drill the cast iron flywheel to get the factor higher than 72 %, but thought about what I was doing and stopped. I do not think the threaded steel plug will ever come out, but I would have preferred to replace the flywheel with a steel one. The crank weight is quite OK. The throttle response is improved by using leaner needles in the carbs and close ratio gears, with high overall gearing. When I started racing the bike, I already had enough experience to make it go. The symptoms for the required changes are often subtle.

With lean needles in the carbs, my bike accelerates much faster as I wind the throttle on. I would not have expected there to be much difference.