A valve lapping tool is a simple hand tool used during cylinder head rebuilds. It consists of a wooden or plastic dowel with suction cups on the ends. You apply a thin layer of abrasive valve lapping compound to the valve's sealing face, insert the valve into the cylinder head guide, and stick the suction cup to the valve head. By rolling the dowel back and forth between your palms while lifting and dropping it, you grind the valve and seat together to create a matching, gas-tight seal.
This process removes minor pitting, carbon marks, and imperfections on the metal faces. When finished, you should see a clean, dull-gray ring around both the valve and the seat. Be sure to clean every trace of lapping compound off the head and valves when done. The abrasive grit is designed to cut metal. If any paste gets left behind, it will enter the engine oil and quickly wear out the cylinder walls and piston rings.