5 series front disks

Eh! Discs wear thin not smaller. OD is about 335mm depending on model.

If you want to check them for wear, it's the thickness that matters. The minimum is stamped on the disc hub. If this is the first change it sounds like you are not hard on brakes. But anyway, most last for two sets of pads. But estimating how long the discs may last is guesswork, and garages tend to load the cards on their side if you're paying.

If you don't have a micrometer measuring them can be tricky due to the lip on the outside. One way is to use twist drills of known size (say 3mm) as spacers and then used a vernier caliper which can be bought quite cheaply. Subtract the sum of the drill sizes from the reading.

It's one of these jobs you really need to do yourself (it's pretty easy) or use an independant. In the UK I saved about 400 gbp over dealer price when changing them all myself using OEM parts and it was an easy morning's work including collecting them.

You can get a cheap-o, Chinese, inside micrometer for about $20 which will certainly be good enough for this use and all but the most precise of measurements.

It isn't the diameter that is important, it's the thickness. With only 20k miles on the car, I have to wonder why you need any brake service at all ...

What Fred said.

Don't mess around with drills as spacers, just buy the right tool for the job.

You should be able to get another 20k out of rotors. I usually just change the rotors with every pad change just cuz they aint expensive if you DIY.

I'm not surprised, as the last set of front pads on my 328i only lasted about 25k with mostly city use. With 90% highway driving, my Z3 managed over 68,000 on the OEM brakes. Like they say, YMMV!

Tom K.

Indeed, but micrometers can require some skill to read. A digital readout caliper probably has more other uses too.

Have you actually tried my method? I merely offered it as an alternative to a micrometer as a digital readout caliper is a much more versatile tool and easier to read. It's not of course as accurate but good enough for this job.

No. I got the right tool.

I also have the ability to read a micrometer without taking it off of the material I am measuing.

You had to buy a micrometer for this task?

Just as well with a disc as you won't be able to remove it. And many people wouldn't have a clue how to read a micrometer correctly.

Sorry you are confused, I should have said, "I've got the right tool."

Exactly. Then, add the complexity of of measuring a drill bit and subtracting the value, and you have a recipe for measuring wrong and doing the wrong math and ending up with an unserviceable rotor on a car.

Buy the right tool and measure properly.

You don't need to measure a drill bit - it's stamped on the shank. In the same way as the disc minimum thickness is stamped on the hub. And if you can't add 3 and 3 together and subtract that from the reading on an LCD display I'd recommend knitting. A dropped stitch won't matter too much.

No - pay the garage to do it.

Usually there's only 2 or 3mm wear allowance so I use my eyes and my fingers.

You should be a brain surgeon with such sensitive, accurate fingers. But unless you carefully remove the rust etc from the unworn lip and have calculated the amount of wear allowed from new, measuring the actual thickness is the preferred way.

Usually there's only 2 or 3mm wear allowance so I use my eyes and my fingers.

I'm not sure that is a safe practice. The specification is for a minimum thickness, and unless you know the maximum thickness I don't see how you can subtract anything to arrive at a value that is worth while. I don't think you can arrive at a decision as to the servicability of a rotor without physically measuring it. If the rotor is obviously toast, then one would not need to measure, but otherwise a measurment is usually required.

I've had several heated discussions with Dave regarding brakes, but this time he is dead-on.

We have a general disagreement on the best method to measure, and we discuss the merits of machining rotors or not, but we agree that one can not make a valuable judgement of the servicability of a rotor without a measuring device of some sort. If one does not know what the New Condition of the rotor is (in terms of thickness) then one can not evaluate the visible wear to determine if the remaining material is sufficient for future service or not.

What happens if you give a 3mm allowance for wear to a rotor that only tolerates a 2mm wear factor? You would end up with a too-thin rotor, that's what. One has to determine the minimum specification, and ensure that this spec has not been worn through. The only way to do that is with a measuring device.

All in the best possible taste, of course. ;-)

Are you joking? If you can't figure out how to read a micrometer you certainly shouldn't be doing your own brake jobs. You might hurt yourself...

Plus they come with "instructions". The may be in Chinglish, but they should tell you what you need to know.