Brake disc minimum thickness: when “still legal” isn’t good enough

Every brake disc has a minimum thickness stamped or cast on it, usually on the hat or the outer edge — something like MIN TH 23. Below that figure the disc no longer has enough mass to absorb heat, and it is due for replacement.

But a disc can be above minimum and still be the wrong thing to leave on a car. Here is what our parts desk tells customers who ring up asking whether they can get away with “just pads”.

How to measure

• Use a disc micrometer or a vernier with pointed jaws. Ordinary flat jaws sit on the wear lip and give a false reading.
• Measure in at least three places around the disc, about 10 mm in from the outer edge.
• Take the lowest reading. Discs rarely wear evenly.

Replace even when it measures OK if…

• There is a pronounced lip. New pads will only contact the centre of the swept area until they bed down, which means longer stopping distances for the first few hundred miles.
• The swept area is scored or blued. Heat spots and deep scoring cause judder and noise that new pads will not cure.
• Rear discs are pitted with corrosion. Common on cars that do short trips. Corrosion reduces the effective friction area even if the disc is thick.
• It is within 1 mm of minimum. A set of pads will take it below minimum well before the pads are worn out.

Rule of thumb from the desk: if you are fitting the second set of pads to the same discs, fit discs too.

Fitting tips

1. Clean the hub face back to bare metal. A few tenths of rust on the hub becomes measurable run-out at the disc edge.
2. Check run-out with a dial gauge if the customer complained of judder. Anything over about 0.05 mm is worth investigating.
3. Lubricate slide pins and pad abutments with high-temperature brake grease — never copper grease on friction surfaces.
4. Bed in with eight to ten moderate stops from 40 mph, without coming to a full stop and holding the brake on hot discs.