Having just posted an update on the frame registration concerns I had, I was reminded about the engine I will be using and how the engine balance method I used could help those with A10 vibration issues. We hear about 54% and 58% balance factors, but every alteration requires a complete engine strip. Well, not necessarily. Here’s the trick: remove sump plate. Turn engine until pistons are at bottom. Now the flywheel bolt should be in the centre of the crankcase hole. Mark a spot about one inch infront of that bolt and another one inch behind. Mark two more, about one inch infront and behind the first two marks. Now drill those marks 9mm dia to a depth of 12mm. Now open those holes to 9.9mm (10mm will do) and tap to 7/16 UNF. Now make up plugs from 7/16 HT bolts to fit those holes, such that they do not protrude (cut a decent screwdriver slot across them before fitting). Now you have balance weights you can fit / remove until you reach the best balance compromise to suit the rev range you prefer – all without stripping the engine.
Below is an example from the crankshaft I made for my vee twin project. Crank is nitrided EN40B and the flywheels – drilled and tapped for balance weights – are mild steel. When you have an engine where the balance factor can at best only be a guess, it would be tiresome to have to strip the engine every time a different factor was tried.
