Norton balance factors can be confusing and range from 52 to 85%. Part of this wild variance problem is due to not specifying “wet” or “dry” – including or not including the weight of oil in the crank.
The best way to think about crank balance is to understand that with a 100% BF the Norton has perfect balance at TDC when the counterweight is opposite the rod/piston. But then the Norton is way out of balance when the piston is at the 1/2 way point of its stroke with the heavy…