not all B series are compatible with eachother, the B20a5 and B21a1 virtually have NO interchangeable parts with other B series motors, they dont even mount up the same so they are hard to swap into other cars.RedLine42 said:Yes B series are compatable with eachother. Your going to need to build the B20 block with pistons, rods ect... The B20s arent going to last long with a stock bottem going past 8krpms.