How big should the bolts be for the sierra prop bearing in the middle weve used 8mm but i think theyll need to be at least 10mm ??any ideas??

Cheers Tim

mine are 12mm.

HTH
Colin

M12 definitely, you don't want it to move!

I've used 10mm. There isn't that much strain on them really, especially if you have a nearly straight run. I'd be comfortable using 8mm HT (12.9) cap screws