How is crankshaft straightness checked and what readings indicate replacement?

How is crankshaft straightness checked and what readings indicate replacement?

• A. Visual inspection for straightness.
• B. Use a straight edge only to check straightness.
• C. Use a dial indicator to measure bend/runout; out-of-spec indicates replacement.
• D. Inspect for color wear on the journals.

Answer: (C)

Crankshaft straightness is checked by measuring how much the shaft deviates from a true straight axis as it is rotated. The most reliable way to do this is with a dial indicator, because it quantitatively records small departures in alignment. In practice, the crank is supported so it can rotate in a fixture (often on suitable supports near the main journals), and a dial indicator is set against a fixed reference surface on the crank. As you rotate the crank through a full 360 degrees, you watch the indicator and capture the highest and lowest readings. The difference between those readings is the bend or runout. This measurement is then compared to the engine manufacturer’s specified limit. If the deviation is out of spec, the crankshaft is considered faulty for straightness and should be replaced (some cases may allow reground/in-service correction only within tight limits, but generally if out-of-spec, replacement is required). Visual inspection or using a straight edge alone isn’t sufficient because they can miss small, nonvisible bends or twists and won’t quantify deviation accurately. Inspecting for journal color wear doesn’t assess straightness at all.

Crankshaft straightness is checked by measuring how much the shaft deviates from a true straight axis as it is rotated. The most reliable way to do this is with a dial indicator, because it quantitatively records small departures in alignment.

In practice, the crank is supported so it can rotate in a fixture (often on suitable supports near the main journals), and a dial indicator is set against a fixed reference surface on the crank. As you rotate the crank through a full 360 degrees, you watch the indicator and capture the highest and lowest readings. The difference between those readings is the bend or runout. This measurement is then compared to the engine manufacturer’s specified limit. If the deviation is out of spec, the crankshaft is considered faulty for straightness and should be replaced (some cases may allow reground/in-service correction only within tight limits, but generally if out-of-spec, replacement is required).

Visual inspection or using a straight edge alone isn’t sufficient because they can miss small, nonvisible bends or twists and won’t quantify deviation accurately. Inspecting for journal color wear doesn’t assess straightness at all.