FAA Aviation Mechanic Powerplant (AMP)Reciprocating EnginesMedium
A technician measures the distance from the centerline of a crankshaft main journal to the centerline of the connecting rod journal (the throw) and finds it to be 2.5 inches. What is the piston stroke of this engine?
- A7.5 inches
- B2.5 inches
- C5.0 inches
- D1.25 inches
Show answer & explanationAnswer & explanation
Correct answer: C. 5.0 inches
Piston stroke equals twice the crankshaft throw because the rod journal moves through a full diameter (top and bottom of its circular path) as the crankshaft rotates 360°. Stroke = 2 × 2.5 in = 5.0 inches.
Why the other options are wrong
- A. This results from incorrectly multiplying by 3, not 2.
- B. This is the throw itself, not the stroke, which is double the throw.
- D. This would be half the throw, an incorrect inverse calculation.
Crankshaft Throw and Stroke
The throw is the offset distance between the main journal and rod journal centerlines; stroke equals twice the throw since the piston travels the full diameter of the crank circle.
- Stroke = 2 × throw
- Throw is also called crank radius
- Determines piston travel distance in the cylinder
Memory trick: Throw is the RADIUS, stroke is the full DIAMETER of the crank's circle.