Definition
A class of mechanical fit between two mating parts, such as a shaft and a bearing, in which the inner part is sized slightly smaller than the hole so the parts can rotate or slide freely against each other with a small, controlled clearance and room for a film of lubricant.
Plain English
A running fit is when two parts are sized so one can spin or slide inside the other smoothly, with just enough gap for oil to get in between them.
Context Anchor
Seen in aircraft maintenance when checking shafts, pins, sleeves, and other parts that must move against each other.
Derivation
Called a 'running' fit because the parts are designed to run — that is, move continuously against each other — rather than be held tight. The clearance is what allows the running motion without binding.
Why Pilots Care
Running fits are what allow engine and accessory parts to keep turning without seizing. If clearance is too tight the parts gall or lock up; too loose and they wear quickly, lose oil pressure, or vibrate. Mechanics check these tolerances during overhaul, and the results affect engine reliability.
Intuition Check
Running fit does not mean the part fits only while the engine is running. It means the fit is loose enough for controlled movement between the parts.
Example Sentence 1
The overhaul manual specified a running fit between the camshaft and its bearings, with a clearance of a few thousandths of an inch.
Example Sentence 2
Bearings installed with a running fit let the crankshaft turn without drag.