Definition
An overhaul of a reciprocating engine's cylinders and valve assemblies without disassembling the crankcase or disturbing the crankshaft, connecting rods, or bearings. The cylinders are removed from the crankcase and inspected, repaired, or replaced, and the pistons, rings, and valves are serviced before the cylinders are reinstalled.
Plain English
A repair job on the upper part of a piston engine -- the cylinders, pistons, and valves -- without taking apart the bottom end where the crankshaft lives.
Context Anchor
You may see this term in engine logbooks, maintenance estimates, or discussions about low cylinder compression, high oil use, or worn valves.
Derivation
Top' refers to the upper portion of the engine -- the cylinders sitting above the crankcase. 'Overhaul' comes from the nautical term meaning to thoroughly examine and repair. So a top overhaul is a thorough repair confined to the top end of the engine.
Why Pilots Care
It restores compression and power at far lower cost and downtime than a full engine overhaul while still meeting airworthiness requirements.
Intuition Check
Do not assume “top” means the physically highest part of every engine. Here it means the cylinder-and-valve portion of a piston engine, not the whole engine.
Example Sentence 1
After the compression check showed two weak cylinders, the owner authorized a top overhaul rather than waiting until the next major.
Example Sentence 2
Following the top overhaul the engine ran smoothly through the next annual inspection.