Definition
A reciprocating engine cylinder design in which both the intake valve and the exhaust valve are located in the cylinder head, directly above the piston. This is the standard valve arrangement used in modern aircraft piston engines and is also called a valve-in-head or overhead-valve cylinder.
Plain English
A cylinder where both valves sit in the top, above the piston, rather than off to the side. Almost every aircraft piston engine flying today uses this design.
Context Anchor
Seen in aircraft engine descriptions, maintenance manuals, and discussions of piston engine cylinder and valve design.
Derivation
The name comes from the shape the valves and piston make when viewed from the side. With both valves in the head and the piston below, the airflow path roughly resembles the letter 'I' — straight down through the intake valve, into the cylinder, and straight up through the exhaust valve. Older designs were named the same way: 'L-head' (valves beside the piston, forming an L) and 'F-head' (one valve in the head, one beside, forming an F).
Why Pilots Care
I-head construction allows higher compression ratios, better breathing, and more efficient combustion than older side-valve designs. It is the reason modern aircraft engines can produce the power and efficiency they do, and it is the configuration described whenever maintenance literature refers to 'the cylinder head' on a Lycoming or Continental engine.
Intuition Check
Do not read “head” as a person’s head or as the front of the engine. Here, the cylinder head is the top part of the cylinder, where the valves are installed.
Example Sentence 1
All current production Lycoming and Continental aircraft engines use I-head cylinders, with both valves operated by pushrods from a camshaft in the crankcase.
Example Sentence 2
I-head cylinders allow better breathing and higher compression ratios than older side-valve designs.