|The MGA With An Attitude
MACHINING Brake Drums and Rotors - BT-108
On 10/7/2009, William Miller wrote:
"Do you have any information concerning the allowable amount of material that can be removed from a brake rotor before it needs to be replaced?"
Back in the day, the Brits were never big on the idea of turning brake drums or rotors, just replace with new, so this specification was never published. My best reasonable answer for brake drums would be +0.040" diameter for rear drums, or 0.060" diameter for front drums.
Reason on drums is because circumference increases as Pi x Diameeter. For 0.040 x 3.14 = 0.126". Beyond that the snail shape adjuster on the rear brakes may run out of travel, in which case you wouldn't be able to adjust the brakes. Front drum brakes with two adjusters may be a little more tolerant of oversize drums.
For brake disks 0.040" total (both sides combined. This information comes from a Confidential Service Memorandum MG 313. Reason on discs is a matter of strength. Get too thin and they may crack, or well worn pads may slip out of their nests in the calipers. MG Midget disks cannot be turned at all, because they start thin (to be cheap). Size tolerance there is -0.010" total, which is only good for moderate wear, no re-machining allowed.