Definition
The small difference, typically about one degree, between the face angle of a poppet valve and the seat angle of the valve seat in a reciprocating engine cylinder. The valve face is ground to a slightly steeper angle than the seat so that initial contact occurs along a thin line at the outer edge of the seat, ensuring a tight, positive seal as the valve closes.
Plain English
When a valve closes against its seat, the two surfaces are not cut to exactly the same angle. The valve is shaped about one degree steeper than the seat. That tiny mismatch makes the valve press against a narrow line first, which gives a better seal.
Context Anchor
Seen in aircraft reciprocating engine maintenance when inspecting, grinding, or fitting poppet valves and valve seats.
Derivation
Interference here means the two surfaces deliberately don't match — they 'interfere' with each other at a single line of contact rather than meeting flush across the whole face. It is not interference in the sense of obstruction or trouble; it is engineered mismatch.
Why Pilots Care
A correct interference angle is essential for proper valve sealing, heat transfer from the valve to the seat, and long valve life. If the angle is wrong after a grind, the valve may leak compression, run hot, or burn — leading to cylinder problems and loss of power.
Grounding Statement
The key idea is controlled contact: the valve should first touch the seat in a narrow, even ring, not across the whole surface at once.
Intuition Check
Do not read “interference” here as unwanted disruption. Here it means a deliberate angle difference that helps two engine parts seal correctly.
Example Sentence 1
During cylinder overhaul, the technician verified the interference angle by checking that the valve face was ground one degree steeper than the seat.
Example Sentence 2
The maintenance manual requires the interference angle to be restored whenever valve seats are ground.