Which wear condition could cause engine noise to become less pronounced as the engine reaches operating temperature? (Repetition for assessment balance)

• A. Worn crankshaft main journal bearings
• B. Worn crankshaft rod journal bearings
• C. Worn camshaft bearings
• D. Worn piston skirts

Answer: (D)

Piston slap is the clue. When piston skirts wear, the clearance between the piston and the cylinder wall increases, allowing the piston to rock slightly and slap the bore, producing a noise that’s most noticeable at low speed or cold starts. As the engine heats to operating temperature, the metals expand and the piston expands more than the bore, which reduces the effective clearance. The lubricating oil film also stabilizes the motion, dampening the slap, so the noise becomes less pronounced. Other wear types—like main or rod bearings or cam bearings—don’t have this temperature-dependent reduction in audible noise in the same way, so they aren’t the best fit for a condition that quiets with engine warmth.