Here's the bigger problem: The valve-to-piston clearance was too tight thanks to the head milling. The intake valves got as close as 0.013 to the piston at 10-25 degrees ATDC, and the exhaust was 0.020 at 10-25 degrees BTDC. Hughes recommended 0.080 on the intake and 0.100 for the exhaust in this application. Here's how we fixed it: First we marked the pistons with a Marks-A-Lot, then installed the heads with checking springs on them. We brought each piston to TDC, then pressed the valves into light contact with the pistons to mark them. Once the head was removed, we drew the shape of the valves on the pistons, then used a carbide bit to hand-cut valve reliefs. During grinding, the engine was sealed in a plastic garbage bag with a duct-taped hole only around the area to be ground.