Question 11 3AE01 - Third Assistant Engineer

The Correct Answer is A. **Explanation of Option A (Correct Answer):** Bearing "crush" (also known as "spread" or "nip") is a deliberate engineering feature designed into split-shell engine bearings. When the two halves of the bearing shell are clamped together in the housing (e.g., in the main bearing saddles or connecting rod bores), the bearing is manufactured slightly thicker across the parting line than the bearing bore itself. This intentional interference fit, or "crush," ensures that: 1. **Positive Seating:** The bearing shells are forcefully pressed against the bearing bore circumferentially, preventing any movement, spinning, or chattering within the housing under dynamic load. 2. **Optimal Heat Transfer:** The tight, continuous contact guarantees efficient transfer of heat generated by friction away from the bearing surface and into the cooler engine structure, maintaining acceptable operating temperatures. Therefore, "crush" is essential for **positive seating of the bearings in their housings**. **Explanation of Incorrect Options:** **B) above normal operating temperatures:** This is incorrect. Crush actually helps maintain normal operating temperatures by facilitating excellent heat dissipation (transferring heat away from the bearing into the block/housing). If crush were insufficient or absent, poor seating would lead to poor heat transfer, which *could* cause overheating and bearing failure (spinning). **C) damage to the journals:** This is incorrect. Damage to the journals (the rotating part of the crankshaft) usually results from a lack of lubrication, debris, or failure of the bearing material itself—not from the inherent design feature of crush. Crush ensures the bearing stays in place to protect the journal. **D) damage to the bearings:** This is incorrect. While excessive crush could theoretically distort the bearing bore slightly, the standard, engineered amount of crush is necessary for the bearing's longevity and proper function. The absence of crush is far more likely to damage the bearings (leading to fretting, spinning, or fatigue failure) than the presence of the correct specified crush.