Couple of weird things while replacing front pads & rotors

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FRUNKenstein
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Couple of weird things while replacing front pads & rotors

Post by FRUNKenstein » Mon Mar 26, 2018 2:59 pm

Just replaced the front pads and rotors on the '87 928S4 yesterday. I noticed the front end shimmy when braking back when I bought the car (October 2014 and just now replaced them :o ). I figured the rotors were warped. I noticed two really strange things.

First, both the pads and the rotors were barely worn at all, as in I'll bet they didn't have more than 5,000 miles on them (I've barely driven the car since I bought it). I thought I had just wasted the cost of new pads & rotors because it must have been something else causing the shimmy. But, the shimmy is gone after replacing them. So, why would a nearly new set of pads/rotors behave like that? Well, I noticed that in between the rotor faces, in the channels, there was a tremendous amount of rust.
928 rotors.jpg
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When I was using an impact screwdriver to loosen the rotor set screws, there must have been 200 or more huge flakes of rust come out of these channels on each rotor. I've seen rust on brake rotors before - the faces and top hat portions. But, I've never seen that kind of corrosion on the insides of a brake rotor. It was amazing and obviously the reason for the shimmy when the brakes were applied.

The second thing was on the inside of the caliper on the driver's side, the slot for the inside brake pad was too narrow. My son and I were doing the project, and I had him try to insert the new pad as I figured it must be something I was doing wrong (although not much to sliding in a new brake pad on a Porsche caliper). I tried all 4 of the new brake pads and they all were just a little too long. Then, I looked at the old brake pad that came out of that slot and noticed hammer marks all along the top edge of the brake pad backing. So, the last person who changed that brake pad apparently pounded the crap out of it to get it to go into the caliper. Rather than following suit, I grinded off a little of the pad backing plate on both ends on my bench grinder. Worked fine. I looked to see if there was any corrosion or dirt in there that was making the slot smaller, but I guess it is just that slot on that caliper is a couple of millimeters narrower than it is supposed to be.

I swear, in every job, there is one PITA surprise lurking: frozen bolt, stubborn electrical connector, whatever. But, it almost never fails that something makes an easy job tougher. At least in this case, there was an easy fix once the problem was identified.
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Current:
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1987 928S4, Guards Red
1987 951, Guards Red
1973 914 2.0 Bahia Red

2006 955S, Lapis Blue
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1987 Bertone X1/9, Verde Chiaro
Gone but not forgotten: 1975 914 2.0, Laguna Blue; 1999 996 C4 Aerokit Black; 1990 964 C2 Guards Red; 2006 955S Arctic Silver

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Re: Couple of weird things while replacing front pads & rotors

Post by B3DAWG » Mon Mar 26, 2018 4:53 pm

I feel ya pain KC. You just never know what you'll get into once you begin a project like this. Talk about frustration... I ordered rear rotors and pads for my Volvo S60. Waited week or so to receive them. Got the car jacked up, removed the tires, calipers, e-brake system, and rotors only to find out they sent me the wrong rotors. UGH!!!! They were slightly different. My rotors weren't that bad, and so I said screw it, installed the new pads and moved with life. Oh, and the place didn't respond to my email about the wrong part. Had to file a claim in eBay to get the return authorized. Sometimes it would have been better to pay a little more and buy them direct from my local dealership. If fact, that's what I did on my daughter XC60. In fact, Athens Volvo matched the online prices I was finding. hahaha Oh well, lesson learned.

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Re: Couple of weird things while replacing front pads & rotors

Post by DTMiller » Tue Mar 27, 2018 10:53 am

Having to massage pads into a caliper isn't all that unusual, especially in older cars. Lots of people grind or trim pad backs to make stuff fit.

That you had front end weirdness that is now gone could be from the rust and gunk deposits, especially if it was noticeably unbalanced in its deposition patter; for example if it sat in one spot for a long time and crud built up on one side you could get the same effect as from a badly unbalanced tire.
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Re: Couple of weird things while replacing front pads & rotors

Post by FRUNKenstein » Tue Mar 27, 2018 2:18 pm

Yep, I agree with you on the rust, crud, etc causing the shudder.

On the brakes, though, it still seems weird because is wasn't the pads that were the problem, it was the caliper. If one of the pad backing plates were a couple of millimeters off, I wouldn't have thought anything of it because they are consumables and I didn't lay out big bucks for the highest priced pads. But, the inconsistency is in the caliper. Back in 1987, that car was one of the most expensive cars on the planet. And a brake caliper is a pretty critical component. So, you'd think the manufacturing tolerances would have been pretty tight. Apparently not.
Not good enough.gif
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www.kansascityautomuseum.com
Current:
2002 996TT X50, Guards Red
1987 928S4, Guards Red
1987 951, Guards Red
1973 914 2.0 Bahia Red

2006 955S, Lapis Blue
Other toys:
1988 BMW 325i Cabriolet, Alpin Weiss
1987 Bertone X1/9, Verde Chiaro
Gone but not forgotten: 1975 914 2.0, Laguna Blue; 1999 996 C4 Aerokit Black; 1990 964 C2 Guards Red; 2006 955S Arctic Silver

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