Yes we thought that the spinner balance would not be to much of an issue as the mass of the spinner is close to the centre of the engine and as you say nothing like an out of balance prop. However two different balanced props and the vibration is still there. The plane was supposed to have a 20 engine and it has a 29 OS engine. Is it possible that the larger engine could itself be causing the problem? Surely if the larger engine is not run flat out and the plane is balanced the larger engine would not be an issue?? Apart from the severe vibration the plane flies really well.