Caliper bolts

[Rob]

This past weekend, I changed the pads and rotors on a friend's '99 M3 (we have no less than four older M3s in our office building of about 40, two 330s and a 325). One of the caliper bolts on the right front wheel was a bitch to get off. It took about half an hour of fighting with it and it laughed at an impact wrench.

When I reinstalled it, I put the bolt on with an impact wrench and then got it as tight as I could with a breaker bar. My torque wrench won't fit in the space, so I didn't try and torque it properly, I just tried to get it (very) tight. I put some anti-sieze on the bolt.

That bolt fell off the next morning. What did I do wrong? Is it possible if bolts are over torqued that they become damaged? I don't think I did anything different than I have the dozens of other times I have changed brake pads. Yes, I swapped the rotors as well this time around, but I can't see how that makes any difference at all.

[The HACK]

Caliper guide bolt? They're rated/spec'd to be 30nm, roughly 22 ft-lbs.

Using an impact wrench or breaker bar on it would stretch out the thread causing it to back out easily. You need to get new caliper guide bolts.

I usually put a little bit of anti-seize on items that I service regularly, like the lug bolts and the brake caliper guide bolts, so that they'll be easier to come off the next time.

[Rob]

What? 22 ft-lbs? The bolts that hold the cliper to the suspension? The bolts that allow you to remove the caliper from the rotor? 22 ft-lbs? I must not be describing it properly. They have always been more difficult to remove then 22 ft-lbs.

If it is the right bolt, though, that would explain why it fell off. We eventually added a cheater pipe to the breaker bar to get enough torque to remove it.

[The HACK]

What? 22 ft-lbs? The bolts that hold the cliper to the suspension? The bolts that allow you to remove the caliper from the rotor? 22 ft-lbs? I must not be describing it properly. They have always been more difficult to remove then 22 ft-lbs.

If it is the right bolt, though, that would explain why it fell off. We eventually added a cheater pipe to the breaker bar to get enough torque to remove it.

Those are CARRIER bolts, as in the bolts that ties down the caliper carrier to the suspension. They should require 16mm sockets to remove? The guide bolts are the ones requiring a 9mm hex key to remove and is in a long, cylindrical shape. You do not need to remove the carrier bolts to replace pads, but you will if you're replacing rotors.

Those are torqued to 120 nm, or 88 ft-lbs for the rear and 140 nm, or 95 ft-lbs for the front. And yes, they WILL back out of the threads are stretched, but they seem to back out way too easily from your description. :?

[Rob]

Good. I thought I had been doing this wrong for years. Yes, I was talking about the carrier bolts. Sorry I don't have all the terminology straight. This one particular bolt was on way tighter than 95 ft-lbs. As I mentioned, we ended up using a cheater to increase torque on a breaker bar and still had to set up to put lots of force on it. If threads will stretch, I should have known that it was a danger when I took it off and just replaced the bolt.

At least I wasn't making a mistake in the process.

Speaking of process, I have never changed pads without removing the carrier bolts b/c that's the way I was taught to do them (with the exception of 1960s era Mercedes pads - they came right out of the back of the calliper after the guide bolts were removed). Where are these caliper guide bolts and how do you get to the pads after you have removed them? I noticed the Bentley manual had different instructions for the pads than I was used to, but I couldn't make sense of the pictures compared to the car.

[Rob]

Oh yeah, thanks for the help, Hack. It was disconcerting to get the phone call on Monday morning from my now stranded friend . . .

[The HACK]

The guide bolts are underneath two black plastic dust shields, you can pop the round cap off to access the guide bolt. Once the guide bolt is off, you can get the caliper to slide off the bracket and change pads that way...And you won't have to touch the caliper carrier.