During overhaul, a technician measures a cylinder barrel and finds that the diameter near the top (near the cylinder head) is slightly smaller than the diameter near the bottom of the barrel, even though the cylinder is within new limits. What does this design feature accomplish?
- AIt allows for easier honing during the manufacturing process
- BIt reduces piston ring flutter during high-speed operation
- CIt compensates for greater thermal expansion at the hot end of the cylinder during operation
- DIt compensates for crankshaft flex under high torque loads
Show answer & explanationAnswer & explanation
Correct answer: C. It compensates for greater thermal expansion at the hot end of the cylinder during operation
Cylinders are often manufactured with a 'choke' bore, machined slightly smaller in diameter near the top (hottest area near the head) than at the skirt end. Because the top of the cylinder runs hotter and expands more during operation, this choke ensures the bore becomes uniformly round and properly sized once the engine reaches operating temperature.
Why the other options are wrong
- A. Choke bore is a functional design feature, not a manufacturing convenience.
- B. Ring flutter is controlled by ring design and groove clearance, not bore taper.
- D. Crankshaft flex is unrelated to cylinder bore taper.
Choke Bore (Cylinder Taper)
A manufacturing feature where the cylinder bore near the head is slightly smaller than at the skirt, compensating for greater thermal expansion at the hotter head end so the bore is uniform at operating temperature.
- Also called a 'choked' cylinder
- Compensates for uneven heat distribution along the barrel
- Bore becomes round/uniform once at operating temperature
Memory trick: Choke it cold, round it hot — the top starts tight to end up true.