Definition
A maintenance procedure performed on a piston engine in which a valve and its matching seat are mated together using a fine abrasive compound, producing a smooth, gas-tight seal between the two surfaces. The valve is rotated against the seat with the abrasive between them until both surfaces show a continuous, polished contact ring.
Plain English
Grinding a valve and its seat together with a fine paste so they fit perfectly and seal tightly when the valve closes.
Context Anchor
Seen in piston-engine maintenance, especially during cylinder repair, valve inspection, or engine overhaul.
Derivation
From the verb 'to lap,' meaning to polish or smooth two surfaces by rubbing them together with an abrasive. The term has been used in machining and engine work for well over a century and refers to the gentle, repetitive motion that produces a precision fit.
Why Pilots Care
Proper valve sealing maintains compression, prevents power loss, and reduces risk of valve burning or engine damage.
Analogy
It is like using very fine polishing paste to make two matching metal surfaces touch evenly all the way around.
Intuition Check
Valve lapping does not mean wiping a valve clean. It means lightly wearing the valve and seat together so they seal evenly.
Example Sentence 1
After grinding the new valve seats, the mechanic performed valve lapping to ensure a gas-tight seal in each cylinder.
Example Sentence 2
After valve lapping, a light coat of Prussian blue showed even contact across the entire seat.