Carbon build-up on the valves or valve seats can cause loss of compression or poor combustion. Valve lapping ensures an air-tight seal between the valve and valve seat. This Powerbuilt 2 Piece Double Cup Valve Grinder Set is perfect for hand lapping valves and valve seats. Set includes a 9-inch tool with 1-1/8 inch and 1-3/8 inch cups and an 8-inch tool with 5/8 inch and 13/16 inch cups. The high quality synthetic rubber cups are chemical resistant and built to last. These non-slip cups are groove-locked to the tool s hardwood handle. Every package comes with professionally written usage instructions and photos that take you step by step through proper tool usage. They meet or exceed applicable ANSI standards and have a lifetime warranty.