[- Paul -]

Anyone know how to work out if the brake discs need changing or not when a service light is showing?

[F31-340i]

Yes - you need to find out the min thickness from BMW TIS. My TIS won't run on Windows 7, but I'm sure someone here has it and can have a look if you state your model, year & engine etc.

Min thickness should also be stamped on the disc itself, but maybe hard to read if they are old. As long as the disc is thicker than min*, you can still use it and fit brand new pads (and therefore they'll last another xxxxxk miles). Well worth checking as I fear dealers often recommend changing when not necessary**.

To measure you just remove a wheel and measure the thickess. Can't really be done without removing a wheel.

I can recommend getting a cheap set of digital vernier calipers from ebay - handy for measuring all sorts, including pads and tread depths with the end of the caliper. You can measure discs accurately that are part worn and have lips by using 2p coins on either side of the disc (put the coins alone in the caliper first and then zero them). Micrometer is another option, but not as useful as calipers IMO.

* unless the disc is warped or the lip appears excessive etc. - the latter a bit subjective though.

** the price of the vernier calipers (about a tenner on the 'bay) is easily outweighed by the risk that a dealer will recommend changing too early IMO if you can't check. Happened to me before when it appeared I got bits of an Inspection I (which includes checking pads/discs) from my BMW dealer "for free" during an oil change....when new discs were suggested I politely mentioned I measured them yesterday and they didn't - the conversation moved swiftly on...

[- Paul -]

Cheers mate - think I found the answer here :-

http://www.bimmerfest.com/forums/sho...d.php?t=523439