I have a truvative BB (X-Type) can anyone explain why one side has a smaller diam hole?

Many thanks

ISTR there's something about allowing float in the drive side bearings…

seems odd to me, basically I have play in the BB so need to replace it, I had a spare RF bb and it wont allow the crank through 1 side as it is too small! just not sure what BB I need to order from crc

I thought that Truvativ used a tapered axle so typically other manufacturers BBs are too narrow and the left cup is too big. Sounds like your problem is a different one.

I'm sure i thought that when i put my last BB on. could not make it fit. Bike rubber mallet sorted that out and been workign fine for months.

Might not be the same problem though

Truvativ is GXP not X type,the axle is stepped, so tightening the crank pinches the bearing on that side between the step and crank, in theory making it impossible to preload the bearings to death like you can with X type.
You'll either need a GXP BB or a Hope with adaptors.

Im using a Hope with adaptors, and what really got me was how tight you have to get it to remove any play, compared to Shimano.