2001 E39 530i, chanigng brakes - follow on

No idea what the OE discs are. I would go for a better/harder wearing brand anyway. I use ATE power discs on my 325 and they have been very good.

Yes. As the callipers will be empty and full of air that needs to be got out. The clamp just stops all the fluid leaking out from the resevoir and the ABS system.

THANKS.

So using a brake bleed kit (one man type), I should be able to get all the air out? No need for BMW to inspect?

I've not done a BMW, but recently did my other car which has four pot calipers so rather more work. On it, the pistons - which are chrome plated

- required replacement due to corrosion. Otherwise a straightforward task, but obviously you do need to clean and examine the parts for corrosion or damage before fitting new seals. And I'd do both calipers on that axle - the seal kit will probably be for two anyway.

I've got one of those battery operated tyre compressors and used this to force out the piston. But you *must* use a secure strong clamp to stop it flying out as it might need considerable pressure to move it. The other (messy) way is to pump it out while still on the car.

Take the Haynes back and get a Bentley instead. You won't be sorry, unless you elect to muddle through service with that Haynes.

Normally the thickness lower limit is engraved on the edge of the disk. Remove the wheel, turn the hub until the mark appears, Measure it with a calliper.

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Difficult to do since you won't get the calliper off due to the ridge at the edge. If you don't have a micrometer use the callipers with a known sized HSS twist trill as a spacer. Say about 3mm.

Yup. As long as the fluid hasn't completely drained out you should be good to go after bleeding the brakes with your one man kit.

Say what!!!

It's an easy task to spread the caliper in order to remove it, and its no trouble at all to turn the hub around to locate the specification stamped on it, even without taking the rotor off. And, it's possiblle to measure the rotor without taking the caliper off.

Perhaps it's a language thing. Measuring vernier callipers have usually parallel jaws. So can't get get past the rust rim unless you use some form of spacer. Same with 'normal' callipers where they are curved and could measure the thickness, but have to be removed to do this.

A micrometer is the easy answer.

Why would anybody use a tool with such an obvious flaw in a particular application? The only tool that can be used to measure a rotor is a micrometer or calipers that haven't got parallel jaws. And the micrometer is the first choice when making the measurement on the car with the brake calipers still in place because a micrometer can get into the tight spot to make the measurement. Having said that, I can't imagine why one would want to measure a rotor and NOT consider replacing the pads. I'd be in there with the intent to replace the pads, and measure the rotors while the calipers were out of the way to see if I needed more parts than I was on my way to get.

Indeed. But calipers have to be removed from the work piece to measure their gap - unless they are vernier types, and common ones have parallel jaws. If you're saying there are reasonably priced vernier calipers with curved jaws then ok - but I've never seen such a beast.

That's what I use, but not everyone has a micrometer, or knows how to read one. But parallel jaw digital readout calipers are now pretty cheap and one of those may be used with a couple of twist drills as spacers and some simple arithmetic.

You're confusing me now. ;-)

No, the brake calipers do not have to be removed, there is plenty of room to measure the rotors with the calipers in place.

While it is certainly possible to measure the rotors with the brake calipers in place, why bother? The brake calipers are off to replace the pads anyway, measure the rotors then.

I'm talking about measuring calipers - not the brake calipers.

But I think I'm pissing into snow.

I'm talking about measuring brake rotors with the appropriate tool(s).

It is not a requirement to remove the brake calipers to measure the brake rotors -- but as a practical matter I don't see any particular advantage of measuring the rotors while the brake calipers are still mounted. Of course, there are those among us that have to drive the car they are servicing when they go for parts, and they would want to get as many of the probable parts as possible in the first trip to the store. I have several cars, and I can disassemble one, and get into another for the Parts Run, and this tends to color my repair technique.

Describe the calipers you use to measure disc (rotor) thickness.

He's not talking about the brake calipers. He's talking about the

*measuring* calipers. You know, those things that look like dividers (or a compass?) but with curved legs. You would need to spread the jaws of the calipers to get them over the ridge and of course then you lose the gap you are trying to measure before you can apply it to a scale.

I don't use calipers to measure thickness, I use a micrometer. I use calipers to hold the brake pads, and I don't bother to measure these calipers or the pads. I visually inspect the pads, and replace as needed, and I open the calipers as far as possible before dismounting them so that I can mount them again when the new pads are fitted.

You introduced vernier calipers to the discussion, and said that they require the use of a twist drill or other to create a surface that is wider than the lip of the brake rotor, then subtract the size of the twist drill or other to arrive at the thickness of the brake rotor.

There is no need to use a twist drill or other to effectively shim the rotor to measure around the lip, then subtract the shims to arrive at the thickness of the rotor. With a micrometer, one can measure the thickness of the brake rotors, and it's even possible to do this while the brake calipers are still mounted.

Ah. But that's not what you seemed to say.

Calipers - not brake calipers - are an engineer's tool for one way of measuring thickness They can be internal or external, parallel jawed or even odd leg. You set them to the thickness then remove them and measure that thickness on a rule.

[snip]

Yes. It's a way for those that don't possess a suitable micrometer.

If you possess a micrometer. And know how to use it.

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^^^^^^^^^

Not necessarily because the vernier caliper jaws are tooled in order the burs which are often present on the edge of a sheet don't touch the jaws. Here there aren't burs but a small rim (lip) instead. Normally the gap present at the bottom of the jaws is more than 2 to 2.5 mm. Large enough to compensate the rim heights

There is another reason for this tooling: when closed, the jaws wouldn't be parallel if the corners were in contact.

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And of course, the jaws aren't parallel !! Don't blame me, it's ASCII ;-)))

Regards