I think (without referencing back) we deducted 4 tenths for any overbore. The thought was on the 318 race, if somebody went .030 over, the extra cubes is one thing, but most do it to raise compression. So, on a 318, if my stock 318 is a true 8:1, and somebody goes .030 over but uses good pistons and has 9:5 to1, they gained compression and cubes. That's why we deducted .4 tenths.