If you determine a drum needs refinishing, remove only enough metal to obtain a true, smooth braking surface. If a drum does not clean up when refinished to the maximum refinish diameter, replace it. Removal of more metal will affect heat dissipation and can cause drum distortion.
All brake drums have a maximum diameter cast into them. This diameter is the maximum wear diameter and not a refinish diameter. Do not refinish a brake drum that will not meet the specifications at the end of the section. Instead, replace the drum.
When you refinish a brake drum, resurface the drum to a dimension no more than 0.76 mm (0.030 in) less than the discard diameter. The refinish diameter is the maximum diameter the drum can be refinished to and still allow safe braking action. If you exceed this diameter, the brake drum will wear beyond the discard diameter during normal brake wear.
Always use sharp cutting tools or bits. Dull or worn tools leave a poor surface finish that will affect initial braking performance. Always use vibration dampening attachments when you refinish braking surfaces. These attachments eliminate tool chatter so that you can obtain a better surface finish.
The best speed for refinishing braking surfaces is a spindle speed of 150 RPM. Crossfeed for rough cutting should range from 0.15-0.25 mm (0.006-0.010 in) per revolution. Finish cuts should be made at crossfeeds no greater than 0.05 mm (0.002 in) per revolution.
New Replacement Drum Refinishing
When installing new brake drums, do not refinish the braking surface. These parts are already at the correct level of surface finish.
Balance
During manufacturing, weights are used to balance brake drums. Do not remove these weights.
After you refinish the brake drums, or when maintaining wheel balance is difficult, check the drums for balance. They can be checked on most off-vehicle balancers. If found to be out of balance, replace the drum.
