Brake Drum Refinishing

Fig 4 Eleven Inch Drum Maximum Diameter Identification:

NOTE: All drums will show markings of maximum allowable diameter.
- For example: An eleven inch drum will have a marking of MAX. DIA. 11.090 inch.
- This marking includes 0.030 inch for allowable drum wear beyond the recommended 0.060 inch of drum refacing.


NOTE: Any brake drum sufficiently out of round to cause vehicle vibration or noise while braking, or showing taper should also be machined, removing only enough stock to true up the brake drum.

If the braking surface diameter is within specifications: Drums should be cleaned and inspected for cracks, scores, deep grooves, taper, out of round and heat spotting.
- If drums are cracked or heat spotted, they must be replaced.
- Minor scores should be removed with sandpaper.
- Grooves and large scores can only be removed by machining with special equipment, as long as the braking surface is within specifications stamped on brake drum outer surface.

MEASURING DRUM RUNOUT AND DIAMETER

NOTE: Measure the drum runout and diameter with an accurate gauge.

Maximum Variation
There should be no variation in the drum diameter greater than 0.O9mm (0.004 in.).


Maximum Runout
Drum runout should not exceed 0.20mm (0.008 in.) out of round.
- If the drum runout or diameter variation exceed these values the drum should be refaced.

Maximum Single Cut
For best results in eliminating the irregularities that cause brake roughness and surge, the amount of material removed during a single cut should be limited to 0.13mm (0.005 in.).

Final Cut
When the entire braking surface has been cleaned a final cut of 0.0254mm (0.001 in.) will assure a good drum surface, if the equipment used is capable of giving the precision required for resurfacing brake drums.
- Deeper cuts are permissible for the sole purpose of removing deep score marks.

CAUTION: Do not reface more than 1.52mm (0.60 in.) over the standard drum diameter.

NOTE: If one brake drum is machined, the other should also be machined to the same diameter to maintain equal braking forces.

After a brake drum is machined: Wipe the braking surface diameter with a cloth soaked in denatured alcohol.