Machine valve spring seats for proper installed spring height and spring cup fitment Hardened pilots fit all 3/8" and 1/2" hand drills Cutter bodies and pilots are interchangeable Cutter: Cuts spring seat 0.550" ID x 1.180" OD The M-Eight cylinder head material under the valvespring seat pocket to the top side of the exhaust port is fairly thin even more so on ported cylinder heads.. TOOL CUTTER VALVE SPRING