If an engine valve does not seal completely against the cylinder head, compression will leak past it. Valve lapping compound contains fine grit inside a grease base. You apply a thin layer to the valve face, insert the valve into the guide, and spin it back and forth using a suction cup tool. The abrasive grit grinds away minor pits and carbon deposits, creating a matte ring of contact. Clean off all compound when finished. Any leftover grit will quickly wear down the valve guide.
Mechanic Glossary
Valve Lapping Compound
Abrasive grinding paste used to smooth the mating surfaces of an engine valve and its cylinder head valve seat.
Frequently Asked Questions
Coarse grit compound is used first to grind down deep pits or rust. Fine grit compound is used last to polish the seat for a solid seal.
The valve face and seat should show an even, unbroken matte grey ring around the entire contact circle. If you see dark spots, keep grinding.
No. Titanium valves have a thin protective coating. Using abrasive compound will strip this coating, causing the valve to fail quickly.