Crumpler

Hey guys, I need some guidance here.
I was putting on new rear brake rotors this week, simple upgrade or so I thought. There were no issues prior.
Anyway, after r/r on heat shields and some cleaning, I was putting new rotors on. The driver's side went on -- blip blip and done.
The passenger side, not so much...
First clue was the caliper would not go on. I messed around with the 6mm countersunk screws first. Various adjustments resulted in being able to load caliper but lateral runout was so wonky the rotor won't turn?

Next I looked at the new rotor and checked tolerances, but looks okay, no imperfections that I could see. I even put the old rotor back on but got the same effect.
Hub spins correctly, no strange run out there that I could see, but I don't have real tools for that.

My eyes were starting to twitch by then and I put the tools down and posted.
I don't have a good reason that things would not be in order if it was previously working well (ie turned normally). Was there some previous sweet spot where the disk and the hub radially offset each other that I disrupted?
What should I look at next he asked?
Thanks ahead of time.

FredR

Crump,

It might help if you can explain exactly what you did and what you did not do. New rotors generally are thicker than old ones and most usually fit new pads to new rotors and new dampers. To fit new rotors generally you need to spread the pistons.

Presumably you checked to ensure the seating surfaces of the hub were spotless and not covered with crap that "dropped off" whilst removing the previous disc.

Crumpler

Thanks for subscribing Fred.
So the original intent was to "upgrade" to cross drilled rotors in rear to match front set.
Disassembly was unremarkable, but I did have to press rotors off each hub with #8 screws and tap holes to get them free.
The only thing touched was the heat shields, which I removed and painted, and some cleaning of parking brake components.

I cleaned the hub surface and then applied optimoly in thin layer.
The new rotors are 25 mm, and appear true and lay flat.
The first side assembled with ease with calipers and Hawk HPS pads loaded.
(That is the pic above).

On the second side I put the new rotor on and secured with the two countersunk screws. Then attempted to mount caliper and noticed that there was not enough clearance (by several mm) on the back of the caliper to get it to onto the hub to mount bolts.

With rotor countersunk screws at ten and two o'clock, I could back out the ten o'clock screw and tighten the two o'clock screw -- and change the angle of the rotor enough to mount caliper. But, it would not spin without catching?

Holding rotor on hub with everything off, and spinning, there seems to be visible run out ( distance from edge of rotor to heat shield changes as it turns)...but when looking at hub spin without rotor it looks true.



I will try and get some pics posted tonight. It has to be something easy that I'm missing -- there just aren't that many parts involved.

FredR

Dave,

It is easy enough to turn the rotor with the caliper removed- were there to be a problem you would likely eyeball such. Check the disc to ensure there are no casting flashes or anything else dumb like that. Also make sure the dimensions are identical to the other side. Ultimately you could also do a trial fit on the other side to see if the problem migrates pain that it may be.

As I remember the rear discs are not directional like the front ones not that that would cause a run out.

soontobered84

Did you check part numbers to make sure you didn't get a mismatched set?

docmirror

Remove the caliper.
Turn the rotor around so the hat faces in, and the disk is sticking out.
Put on a couple of lug nuts and cinch down.
Start the car, and put in D(wheels blocked, e-brake off) ONLY if you do not have LSD.
Observe the rotor for lateral run out.
Do NOT apply the brakes.
Shut off the engine.

Report results.

Crumpler

Thanks you guys, those are good ideas. I will be working on that and report back in. Hopefully it's something easy.

Mrmerlin

I will guess that you also need to clean the edge of the hub so the new rotor will properly seat,
if you look carefully you will notice that their are 3 pads on the edges of the hubs,
these need to be cleaned before a new rotor will properly seat.

worf928

What Stan said.

There have been rear rotors sold as 928 rotors that weren't and wouldn't fit. But, since you say it happens with the old rotor too then it can only be two things:

1) rotor isn't seated fully
2) the hub isn't in the same location relative to the hub carrier as it was when you pulled the rotor.

Unless you've recently pulled your 1/2-shaft or done the bearing on that side it's unlikely to be #2.

Crumpler

Hey guys, I very much appreciate your help.

I got out in the garage tonight and cleaned the hub aggressively, and then tried to set the new rotor again. This time I noticed that it would not seat completely, about 3mm space between hub and rotor when tight.

I cleaned the old rotor and it went back on easy.
I then pulled the new rotor that fit on the other side and put it on this side and it also works fine.
Bottom line, QC issue on one out of two of the new rotors.
I guess I learned my lesson on this one, it was purchased on a "bargain" website and not from our well known vendors.

FredR

Dave,

Time to ask for a replacement [stating the bleeding obvious]. At least you now know where the problem lies. Hopefully the purveyor will do the decent thing.

Oh and before you castigate yourself too much sometime around the 2000 mark Porsche were having significant quality control issues- they went to Toyota for help [or so I understand]! Porsche quality control was far from perfect same logic applies to suppliers I suppose- one error does not make all item garbage. I have had Zimmerman discs on my rear end for 12 years wiht no problem but I have always used Porsche supplied discs up front- needless to say gwtting discs sent over here costs a fortune so negates the Porsche mark up.

Crumpler

I hear you Fred!
Yeah, I called the vendor today and told him they were taking them back. He seemed confused. After hearing the situation he asked if I had used them...
​​I asked how I would have used them if they did not fit....
​​​​​
Also told him i had seen enough and was going back to Zimmerman.

SeanR

Good post Crumpler. Go with the guys that know what they are selling and you shouldn't have any issues.

Crumpler

Ok I'm 58% serious....
Why do they look so good as a hovercraft?!