Definition
A deliberate manufactured taper in an aircraft engine cylinder barrel where the upper end (near the cylinder head) is ground to a slightly smaller internal diameter than the lower end. When the engine reaches operating temperature, the cylinder head expands more than the barrel, and the choke straightens out, producing a uniform bore that allows the piston rings to seal correctly against the cylinder wall.
Plain English
The top of the cylinder is built a tiny bit narrower than the bottom on purpose. It looks unequal when cold, but once the engine heats up and the metal expands, the inside of the cylinder becomes the same width all the way down, which is exactly what the engine needs to run properly.
Context Anchor
Seen in aircraft engine maintenance, especially when inspecting, measuring, or overhauling engine cylinders.
Derivation
The word 'choke' comes from Old English 'aceocian,' meaning to constrict or narrow. In engineering it has long been used for any deliberately narrowed section. Here it describes the narrowed upper portion of the cylinder bore.
Why Pilots Care
Correct choke maintains proper piston-to-wall clearance when hot, preventing scoring, excessive oil consumption, and possible seizure.
Grounding Statement
The cylinder is built slightly narrower at the top so that when it heats up and the metal expands unevenly, the bore ends up perfectly straight.
Intuition Check
“Choke” here does not mean the engine is being starved of air or fuel. It means the cylinder is built slightly narrower at one end to allow for heat expansion.
Example Sentence 1
The mechanic noted that the smaller diameter at the top of the bore was the choke of the cylinder, not wear, so the cylinder was within service limits.
Example Sentence 2
Loss of the designed choke after many hours of operation can cause the piston to rock and increase oil consumption.