Definition
The two main structural parts of a poppet valve in a reciprocating aircraft engine. The valve head is the large, flat or tulip-shaped disc that seats against the valve seat in the cylinder to seal the combustion chamber. The valve stem is the long, slender shaft extending from the head; it rides in the valve guide and is acted on by the rocker arm to open and close the valve.
Plain English
Every engine intake or exhaust valve has two parts: a wide top called the head that plugs the opening into the cylinder, and a long thin shaft called the stem that the engine pushes on to open and close it.
Context Anchor
Seen in piston-engine maintenance, valve inspection, compression problems, and discussions of intake and exhaust valves.
Derivation
Head and stem are borrowed from everyday shapes — the head is the wide top (like the head of a nail or mushroom), and the stem is the narrow shaft below it (like a flower stem). The names describe what the parts look like.
Why Pilots Care
Proper function ensures the combustion chamber seals correctly during the power stroke; worn stems or damaged heads cause compression loss and power reduction.
Analogy
Picture a mushroom: the cap is like the valve head, and the stalk is like the valve stem. The shapes are not identical, but it helps you see the broad sealing end and the narrow guiding part.
Intuition Check
Do not read head as the top of the valve in every installed position. In this engine context, the valve head is the broad sealing end, and the valve stem is the long shaft attached to it.
Example Sentence 1
During the cylinder inspection, the mechanic found carbon buildup on the valve head and slight wear on the valve stem.
Example Sentence 2
Carbon buildup on the valve head can prevent proper seating and reduce engine compression.