Definition
In a reciprocating engine, the number of degrees of crankshaft rotation before top center or bottom center at which an intake or exhaust valve begins to open. Valve lead allows the valve to be fully or partially open by the time it is needed, improving cylinder filling and scavenging at operating speeds.
Plain English
Valve lead is how early a valve starts to open before the piston actually reaches the top or bottom of its stroke. The valve gets a head start so it is already open when the engine needs it.
Context Anchor
Seen in piston-engine maintenance, valve timing checks, and engine theory discussions.
Derivation
Lead' here is used in its timing sense -- to be ahead of, or come before, something else. The valve 'leads' the piston by opening before the piston reaches the relevant point in its travel.
Why Pilots Care
Correct valve lead allows efficient cylinder filling and exhaust scavenging, protecting engine power and preventing valve damage.
Grounding Statement
Picture the piston still moving toward the end of its stroke while the valve has already started to open.
Intuition Check
Lead does not mean the metal lead here. It means the valve event happens ahead of the piston reaching the end of its stroke.
Example Sentence 1
The intake valve lead on this engine is 15 degrees before top center, so the valve is already starting to open as the piston finishes its exhaust stroke.
Example Sentence 2
Proper intake valve lead helps the cylinder fill more completely on the intake stroke.