My machinist said my core was one of the best he had seen for straightness. It was off by about .005"-.007" across the block. Because of budget constraints, I did not have it square decked off of the crank. I just had it parallel milled which means it's still off by that much. If you're building a low compression motor, that's no big deal. I might have a cylinder at 9.25:1 and another at 9.4:1. On a high compression motor that is very borderline for fuel, you'll have one cylinder at 10.5:1 which is ok for pump gas, but another might be at 11:1+ which will be way into detonation and you won't know about it because all of the other cylinders run fine. Oh yeah, I didn't even need to have the mains line honed. The rear main set up a little looser than the rest at .0025" while the others were .002" for oil film clearance.