1/8 onroad actually.
Our 3.5cc AAC engine has just a 12mm crankshaft with a 9mm hole. To save more weight on the outside there is just an OS shaft and on that we have a centax like aluminium flywheel nut with the 5mm shaft in one piece. Comparing with normal engines we are totally 50 gram lighter
We have runned very small counterweights on the crankshaft but indeed lots of vibrations alhough the performance was very good. With a bigger counterweight the vibrations were less but the performance did not gain much, the lifespan did!