Having finally got my new batch of 82mm M20 pistons made, I'm able to get a weight for the piston assembly.
This has encouraged me to repair the crankshaft.
Today I did a dummy crank build without the conrod mounted.
Very pleased that I managed to true it to 0.0005" Yes half a thou.
Having weighed all the relevant parts, the computer gave me a balance weight to mount on the big end.
I've based it on being in tune at 5000 rpm. I've done this for another M20 and the customer is pleased.
Surprisingly my crank was not as bad as I normally see.
5 grams on one flywheel and 9 grams on the other.
An idiot test with plasticine proved the results.
The phase error was only about 30 degrees.
In reality this would have ran quite smoothly.
Still not good enough for me, I'll get it to 1 gram.
I'm not sure I'd want to rev an M20 to 5000 rpm ! But I'm completely with you on accuracy of trueing the flywheels, and of dynamic balancing. I had the crank in my 350 AJS rigid trials bike done when I built the engine about 10 years ago. I was surprised at how much material was removed from different places on each flywheel. Its done an awful lot of work in those 10 years and is still as smooth as ever. My WD G3l Matchless also got its crank balanced, and piston skirt coated when I rebuilt that one about 18 months ago. Again, a lovely smooth running engine. We now have the time and technology to build these engines to a much higher standard than when they were first built. I can't see a reason not to do it and enjoy the results.