Definition
On a crankshaft, the throw is the offset distance between the centerline of the main bearing journals and the centerline of the crankpin (rod journal). It determines the radius through which the crankpin travels and, when doubled, equals the piston stroke length.
Plain English
How far the crankpin sits off-center from the crankshaft's main axis. The bigger the offset, the longer the piston travels up and down each time the crankshaft turns.
Context Anchor
Seen in aircraft engine maintenance, engine diagrams, and crankshaft inspection discussions.
Derivation
From the mechanical sense of "throw" meaning how far something is moved or displaced from a reference point — like the throw of a switch or lever. In a crankshaft, it describes how far the crankpin is thrown out from the main axis.
Why Pilots Care
Throw directly sets piston stroke, which determines engine displacement and power output. Anyone working on or studying reciprocating engines needs to understand that the crankshaft's geometry — not just its rotation — is what drives the pistons.
Analogy
Think of a bicycle pedal crank. The pedal is not in the center of the wheel axle; it is offset, so your foot can push in a circle. A crankshaft throw works in a similar offset way inside the engine.
Intuition Check
“Throw” does not mean tossing something here. It means an offset part of the crankshaft that moves around the shaft’s center as the engine turns.
Example Sentence 1
The crankshaft's throw is half the piston stroke, so a four-inch stroke engine has a two-inch throw.
Example Sentence 2
During overhaul the technician measured the throw on each crankpin to verify it matched specifications.