Definition
The offset cylindrical journal on a crankshaft to which the large end of a connecting rod is attached. As the crankshaft rotates, the crankpin travels in a circular path around the main bearing axis, converting the reciprocating motion of the piston into rotary motion of the crankshaft.
Plain English
The off-center pin on a crankshaft where the connecting rod attaches. Because it sits off to the side of the crankshaft's centerline, it swings in a circle as the crankshaft turns, which is what lets a piston going up and down drive a shaft that spins.
Context Anchor
Seen in piston-engine maintenance, engine teardown descriptions, and discussions of crankshaft or connecting-rod wear.
Derivation
From 'crank' (a bent arm or lever used to turn a shaft) and 'pin' (a short cylindrical rod). The crankpin is literally the pin on the bent part of the crankshaft.
Why Pilots Care
A worn or failed crankpin bearing can cause loss of engine power or complete engine seizure in flight.
Intuition Check
Do not picture a small loose pin that is inserted and removed by hand. A crankpin is usually a machined part of the crankshaft itself, shaped as a round surface for the connecting rod to ride on.
Example Sentence 1
Inspection of the crankpin during overhaul revealed scoring that required the crankshaft to be reground.
Example Sentence 2
Excessive play at the crankpin can produce a rhythmic knocking sound under load.