New rotors & pads - too tight?

[walendo]

Hi folks, I need some more guidance on my '90 K75RT (non-ABS).

A while back, my dealer told me that my front brake rotors had worn thinner than the minimum recommended thickness and I figured I'd spend the time to learn how it all works and replace them myself. I picked up some EBC MD604 rotors, and the recommended FA171 pads to go with them. Then I figured I should rebuild the calipers since I had it all apart, and ordered the BMW caliper rebuild kits.

I tore down the calipers, cleaned them, and put them back together with the rebuild kits. The rebuild kits contain new rings, o-rings, and a dustcap. When I reassembled them, I noted that the dustcap does not sit flat against the caliper. You can push the piston all the way down, but there's a ring of rubber that won't go flat.



Anyway, I continued on my merry way and reassembled the caliper halves. When I put the pads in, I saw that things would be pretty tight - the pads are not able to sit flush with the calipers due to the rubber from the dustcap - but figured this was par for the course.

So, I put the new rotors on the wheel and started putting the bike back together. Well, things are definitely tight. I can get the calipers on and the pads around the rotors, but they definitely rub. No so tight that I can't turn the wheel by hand, but tight enough that it definitely wouldn't spin on its own. And that's with just one side on.

Is this normal occurrence for new pads and rotors and I just need to 'bed' them in?

If not, what did I do wrong and how do I fix it? Different pads? Could it be that the dustcap should sit flat, and since it's not it's pushing the pads too far out? If so, what's the secret to getting it to sit flat?

Or am I missing something else entirely?


Thanks! -w

[motodude]

The back of the pad should/must fit flush against the piston.  That said, the pad will also touch/ride on the rotor.  Not "tight" against the rotor, but there will be some drag.  You should be able to turn the wheel by hand, it should turn relatively easily and smoothly in both directions.  It won't "spin" with the calipers installed.  There should be no, "tight spots".  And, there isn't a lot of "extra" room with new pads and new rotors.

I suspect you just need to push the dust boot down into the caliper half and you can get the pad flush against the piston.  But it is hard to tell from here.

[walendo]

Hmm. Should the backing plate of the pads be able to sit flush against caliper? Right now, as long as the piston isn't all the way in to the caliper, the pad rests on the piston. It's once the piston gets to a certain depth (e.g flush with the caliper) that the pad no longer rests on the piston, but on the rubber rings of the part of the dust cover that surrounds the piston.

Does that make sense? Another way of asking is: if I take the dust cover off, I can push the pad (and the piston) down so they're both flush with the caliper. Should that be the case with the dust cover on? (And, if so... I need to go stare at that dust cover for a while, because it's not clear how it would ever be able to sit flush with caliper seeing as how it surrounds the piston but there is no recess for it directly around the piston).

Thanks, -w

[Inge K.]

Piston(s) is mounted the wrong side in.

Inge K.

[walendo]

Holy cr... awfish. I think you're right, Inge.  That would explain it.

I feel pretty silly at the moment.

[walendo]

Confirmed. I'd put them back in backwards. Took 'em apart, flipped the pistons, and it's all good now. There is even clearance between the rotor and the new pads.

Thanks for your help, guys!