When measuring a brake drum, a technician uses a brake drum micrometer. They measure the drum diameter at several points and find the measurement to be 10.030 inches. The drum is stamped with a nominal diameter of 10.000 inches and a maximum allowable diameter of 10.060 inches. What should the technician do?
- AReinstall the drum, as it is within specifications.
- BReplace the drum, as it is beyond its service limit.
- CAdjust the brake shoes to compensate for the wear.
- DResurface the drum to remove the excess material.
Show answer & explanationAnswer & explanation
Correct answer: D. Resurface the drum to remove the excess material.
The measured diameter (10.030 inches) is greater than the nominal diameter (10.000 inches), indicating wear. However, it is less than the maximum allowable diameter (10.060 inches). This means the drum has enough material remaining to be safely resurfaced (turned) to restore its smooth friction surface. The resurfacing process will remove a small amount of material, bringing the diameter closer to the nominal but still within the maximum limit.
Why the other options are wrong
- A. The drum is worn past its nominal diameter, so it needs service (resurfacing), not just reinstallation.
- B. The drum is NOT beyond its service limit; it is still below 10.060 inches.
- C. Adjusting brake shoes will not correct a worn or out-of-round drum; it will only compensate for clearance.
Brake Drum Machining Limits
The specified minimum thickness or maximum diameter to which a brake drum can be machined or worn before it must be replaced for safety and proper function.
- Nominal diameter: original manufacturing size.
- Maximum diameter (MAX DIA): absolute largest safe diameter after wear/machining.
- Always replace if diameter exceeds MAX DIA or if severely cracked/damaged.
Memory trick: Measure the drum, check the max, if it's in, resurface or relax, if it's out, then axe!