whats the biggest that you can bore a b21 out to? 87mm? 89mm? i believe the stock is a 84mm?

 

depends if you plan to go turbo

anyway with an h22 it's recommended to not go over 89mm for boost and 90mm for na, that's whit a 87mm stock bore

so if taking the same rule of thumb, the MAX would be a 3mm over with 87mm, but that would be pretty hard, you may not keep it that long!!!

 

planning 2 boost it