I also believe in balancing the motor to suit JS rods and pistons.
One reason is this: if using a 50+ year old stock crank, you do not know your starting point, so all the calculations and estimations are based on sand.
I have it on good authority from back in the day that Norton botched the crank balancing on Commandos. My hypothesis is that the isolastics worked so well, they dropped the ball on crank balancing.
Plus, an engine could have been re-balanced by some unnamed person, at some unknown time, using some unknown theories.
Sounds wonderful, but when I was building my engine, I tried to find someone
I’ve built 3 JS equipped engines now and have had them properly balanced each time and the results have been fabulous every time.
The most recent was my Seeley with a 1007 motor. I believe it was balanced incorrectly when first built as the vibration was horrible (ie I thought it would brake things), but then a 1007cc, 11:1cr, solid mounted twin ain’t ever gonna be turbine smooth of course.
I canvassed opinions and got a range from 72% to 78% as the most recommended factor for a rigid mounted Seeley. So, knowing no better, I aimed in the middle and went for 75%. The re-balance and the use of JS rods and pistons TRANSFORMED the bike.
Yes, I know I did 2 big changes at once so cannot put all credit to the JS parts, but they removed over HALF A POUND of reciprocating weight. Just think about the stresses involved with that going ‘up, stop, down, stop, repeat’ over 100 times per second !!
Unsurprisingly, the engine produced more power as a result as well as being much smoother, so def a win-win.
