I am going to build a b20 block what is the biggest bore pistons i can go with sleeving the block that will b ok, i am thinking about 86mm maybe 87mm, i am just thinking that i could, or would it be safer for me to sleeve it with 84.5mm bore

 

why not sleeve a GSR block. I wouldnt waste my money resleeving a B20 block. Or sleeve a LS block. Its cheaper and identical to the B20 except the oil pump. You could go 87mm but there will be so little space between the cylinders that it might not seal. I would go 85mm to be on the safe side.