so from experience I can tell it is going to depend on what oil system you want to use. If you want a 1/2" factory style pick up tube then 4,25 is the biggest crank that will fit in a factory cast block. If you want to use a swinging pickup style in the pan, you can plug the factory hole and you will have to grind clearance right there for the rod to clear but a 4.375 crank will fit. I built a motor that was 4.375 X 4.375. It was a beast.Birdtracker