Definition 1 of 2
Definition
In a threaded fastener such as a spark plug or fitting, the small difference in thread angle between the male and female threads, deliberately designed so that the threads bind tightly when fully seated, providing a gas-tight seal under the high pressures and temperatures of engine operation.
Plain English
The threads of the part and the hole it screws into are cut at very slightly different angles on purpose, so when the part is tightened down, the threads pinch together and form a seal that does not leak.
Context Anchor
Seen in aircraft engine maintenance when inspecting, grinding, or replacing valves and valve seats.
Derivation
From Latin 'interferre,' to strike against. The name describes what the threads do: they 'interfere' with each other slightly when tightened, creating the sealing pressure.
Why Pilots Care
Correct interference angle ensures valves seal tightly, maintain compression, transfer heat properly, and avoid power loss or valve damage.
Intuition Check
Do not read “interference” here as a problem or blockage. In this term, the interference angle is a deliberate small mismatch that helps the valve seal.
Example Sentence 1
The spark plug uses an interference angle on its threads to seal combustion pressure inside the cylinder.
Example Sentence 2
An incorrect interference angle allowed combustion gases to leak past the valve seat during the compression check.