Definition
A specified clearance between the valve lifter and the valve stem (or between the cam and the lifter on overhead-cam engines) used as the reference point for setting the timing of the valves during engine assembly or adjustment. It is not the operating clearance the valves run at in service; it is a temporary, larger clearance used only to establish the precise crankshaft position at which a valve begins to open or finishes closing.
Plain English
A special, temporary gap set between engine parts to help the mechanic find the exact moment a valve starts to open or close, so the timing can be set correctly. Once timing is checked, the gap is reset to the normal running value.
Context Anchor
Seen in aircraft engine maintenance instructions, especially when checking or setting valve timing on a piston engine.
Derivation
‘Valve timing’ refers to when each valve opens and closes relative to piston position. ‘Clearance’ here means a measured gap. Together, the term names the gap used as a measuring reference while setting that timing — distinct from the smaller gap used in normal operation.
Why Pilots Care
Incorrect clearance leads to burned valves, loss of compression, rough running, or engine failure.
Analogy
It is like using a marked starting point on a ruler before taking a measurement. The gap gives the mechanic a consistent reference point for the timing check.
Intuition Check
Clearance does not mean permission here, and it is not an air traffic control clearance. It means a measured mechanical gap used while checking valve timing.
Example Sentence 1
Before checking valve timing on the overhauled engine, the mechanic adjusted the lifter to the valve-timing clearance specified in the manual.
Example Sentence 2
After confirming all clearances, the engine started smoothly with no excessive valve clatter on run-up.