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Looking for advice on best balance factor for street 40-60 mph?

At what revs ?

3-4k

Workshop manual for 1971 says "The balance factor is 70 per cent". Important thing is to make the balancing dynamically with balancing machine.

Yes Im sending it for dynamic balance. I've heard anywhere from 68 to 74. Thats why Im questioning. I think SRM does them to 68 percent.

Ok. My experience is that the percentage is not so important, the main thing is the dynamic balancing. Manufacturers made only static balancing and the results were quite different between each crank. I have balanced dynamically several british bikes cranks and they have all been different. In every case the owner has been satisfied for the result and all have been balanced to the factor what has been written at manufacturers workshop manual. The dynamic balancing is the key.

Static balance for road 70% ( BSA WSM page B44 ), smooths out around 4K, balance for racing 56% ( devimead), smooths out at higher RPMs.
Agreed on dynamic balance, its a good thing..

My lightning is balanced at 62%. I asked for it to be smooth around 3500-4500, and it fairly smooth and im quite happy with it. but dynamic balancing isn't like pressing a switch and everything is ok...

I fitted a full race cam to my motor and it was incredibly smooth, I changed the cam and it has some vibration (not a lot but it was unbeleivably smooth the the FR cam). so... changes to cam timing can make a difference,

Ignition timing will alter vibration to some degree as will ensuring that the carb is jetted correctly (which might not be what the book states) and if you have 2 of them, then that they are in good balance.

Balancing pistons to within fractions of a gram, balancing wrist pins, rods etc etc. will make the biggest difference and that before you send the crank to the balancer.

Dynamic balancing is only as good as the bloke who does it.

+1, there are closer companies to me offering this service but I still use the one I trust

All said, a well built A65 should be pretty smooth at 40-60mph. Dynamically Balanced or not.

I love the way my bike feels between 45 and 55 mph. It's the perfect range for under-stressed and nimble. I wish I had more opportunities to ride in this range where I live but there's usually someone blocking my way doing 39 or highways at 60+

I trailered my BSA out to Andover to attend the annual All-British Rally in Lancaster, and I know exactly what you mean - there are only two kinds of roads in eastern Mass.: Six-lane, average 75 mph highways, or twisty, narrow, 30-40 mph lanes with lots of trees, houses, and driveways.