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I just replaced my first rotor (front) and I was a bit suprised to find that there was a little bit of play in the rotor after I had tightened down the rotor mounting bolt. Given there is only one mounting bolt I can’t see how there wouldn’t be any wobble.
Is this normal or am I in for a big shock?
Thanks,
Andy
Andy, what you call the mounting bolt is simply a guide bolt, also prevents the rotor from moving back and forth under brake pressure. Your lug bolts/nuts will hold the wheel and rotor. Not putting the guide bolt in will permit rotor motion most of the time (even being torqued properly) under braking, sometimes making a small clicking sound when applying or releasing the brakes… Your old rotor did not have any motion because of rust holding it on…
When I was disassembling the other side I discovered that didn’t have a guide bolt at all. The thread on the hub is thoroughly rusted and I can’t get a new bolt in there at all. I’m going to clean it tomorrow and if necessary try and tap a new thread.
I think bmw should have used dowel pins instead of those bolts. The previous owner of my car broke off both of the front bolts. They are hardened steel and unextractable. As for the wobble just make sure that the rotors are fully seated, and the bolt is in tight. I thought I had my rear rotor seated all the way until I was driving home from work the next day and I heard and felt a major wobble on right hand corners. When I took my left wheel off my lug bolts were loose and there was a spot inside the rim where the rotor bolt had worked it’s way into the aluminum rim.
greensha wrote:
The rotor retaining screw can be left off, but juggling a wheel, the rotor, and hub if you aren’t using studs is not not fun. The sole purpose of the retaining screw is to fix the alignment of the rotor and hub.
Run a tap into the hub to clean the threads and there won’t be any problem with the screw.
greensha wrote:
[quote]I just replaced my first rotor (front) and I was a bit suprised to find that there was a little bit of play in the rotor after I had tightened down the rotor mounting bolt. Given there is only one mounting bolt I can’t see how there wouldn’t be any wobble.
[/quote]
It’s normal. It’s the wheel that really holds the rotor on. Once you tighten the wheel’s lugnuts, the rotor should rotate true. If not, then you can worry.
Don’t overtorque that little allen bolt. Finger tight. A little anti-seize wouldn’t hurt too.
Get yourself a set of studs. Will make wheel changes a lot easier, and rotor location a non-issue. Problem solved!
I’ve put studs on. So I don’t need the retaining screw if I have studs? As you can tell, I’m pretty clueless about this whole car mechanic’ing thing… I’m slowing building a library of clues though!!!
greensha wrote:
The retaining allen bolt is handy because it keeps the rotor on while you put the brake caliper back on. I’d continue to use the bolt. Finger tight with some anti-sieze.
Yep a little anti-sieze and you should be able to just hold the rotor with one hand and tighten the screw with your other hand.
Make sure you clean the hub really well with a wire brush to get all the old rust and crap out. I don’t recall my brake rotors not being flush to the hub before I mount the caliper and wheel. Also check in new rotor to make sure there is noting in there keeping it from being flush with the hub.
I’ve had the good fortune of having two of the rotor retaining screws break off when trying to remove them. The last time I went to Napa to get them, the guy at the parts counter refused to order them for me. He swears up and down that BMW only used them to hold the rotors on at the factory when rolling down the assembly line - and are totally unneccessary. Truth or fiction, you decide.
The retaining bolts are standard metric hardware. Any good hardware place will have hardware that will fit.
Yes, the screws hold the rotor to hub for ease manufacturing/assembly. You don’t need them. If you want them for ease of assembly fine, but for goodness sake do not tighten them! As long as they don’t stick out past the wheel mounting face your fine.
If you have 2 screws between hub & rotor it is because the auto company does the final machining of the rotor when it is attached for the hub. Yes, I know that for sure. 
I think I’ll leave them off. It’s only one more thing that will end up getting stuck and causing me more trouble. Right now I’m trying to get the rear driver’s rotor over the parking brake shoes. I had a bugger of a time getting the passenger rear rotor on. The p-brake actuator had seized pretty good.
Andy