3WindCoupe

iTrader: (3)

Here's the issue: We have an 88 GTA posi 9 bolt and we just installed new rotors ( drilled and slotted) and we bought rebuilt calipers (iron Delco-Moranes) and new pads. We can't come close to getting everything back together. The rotor width is proper ( 1.04 ") but the spacing between the pads is only 0.9" even though the piston has been fully retracted ( and, yes I know about how the ebrake works). By fully retracted I mean I still have the clip that holds the inner pad on the piston but it's the only part of the piston that's exposed and I can't retract it further.
I thought maybe I got a thicker than normal set of pads but I've checked over ten others from different parts stores with the same result ( no surprise there - fronts and backs usually have the same part number).

Any thoughts? Does GM or anyone carry a thinner rear pad? Should I just remove 10 - 15,000 miles of pad to get them to fit? How do you do this?

Thanks in advance for any advice.

87350IROC

iTrader: (2)

Can you install the new pads in the old caliper and see if it fits?

Part numbers for everything?

3WindCoupe

iTrader: (3)

Unfortunately, I don't know which calipers were originally on this diff but the old ones I have measure the same opening width as the two rebuilt ones: 2.31" inside width between one mounting ear and the body. That is, visually they appear identical and from this one measurement are the same. What part numbers would you like? Did GM use different D-M castings that, in turn, would cause a change in pad dimension?
Thanks.

87350IROC

iTrader: (2)

Well your problem is not obvious. Regardless of whether they look the same, I would take the 5 minutes to mount the pads in the old caliper to make sure. Its all about ruling out potential causes.

I want part numbers to make sure the parts you have are correct. Calipers, pads, rotors.

Do the pads have thick shims attached?

3WindCoupe

iTrader: (3)

The shims on the pads are thin just like the old pads I happen to have around. However, I have two kinds of old pads. The backing plate on some are about 1/8 inch and others are about 1/16 inch. If I were to combine two of the thinner backing plate pads I will gain 1/8" or 0.125 " and, voila, I could mount them no problem.

Part numbers - I'll forward those in a separate post later this evening.

3WindCoupe

iTrader: (3)

Here are the part numbers:

Rear rotors: AC Delco 18A1042 and 18A1043 (RockAuto)

Rear pads: Wagner ThermoQuiet MX202 (Advance Auto)

Rear Caliper: SLC254 (Advance) or 18-4164 (CarQuest)

I tried using an old caliper as you suggested - I think this is the one that was
original to this diff. Same exact opening between the two pads - 0.92" - as the rebuilt calipers have. Obviously, we need something greater than 1.04" to mount
over the rotor.

I did go through all the old pads I had in my collection of old brake parts and
found two pairs that will work for now so I can mock up the rear assembly and
get on with the build and come back when I score some pads that will work.
Visited the Chevy dealer down the street and although they don't stock anything
the rear part number is different from the front and I'll get to see them and measure them Monday.
In the meantime the only solution I see is to sand the brake pad matrix off. Tried it with 50 grit by hand and it's going to take a long time. I need to find a belt sander - but I've always wanted one. Hmmm.....

87350IROC

iTrader: (2)

Hmmmm, I will have to take a look at my old parts later. I have swapped pads front to back with no issues.

3WindCoupe

iTrader: (3)

As I mentioned, the local GM dealer ordered pads last Friday for an 88 GTA 9 bolt with Delco-Morraine calipers. Unfortunately, the pads that were delivered were for the PBR set-up so no go. Wound up at Home Depot and bought a cheap belt sander and some 30 grit belts. Carefully set it up in my vise and lashed it down with some rope. Took off about 0.1 " (a little from each pad) in half an hour and now one caliper is back on the car. I hope to finish the other side tonight. My only concern is that I don't have the pads sufficiently parallel and square because I just used hand pressure. They look pretty good when I miked them though. I'll swap the fronts for these modified backs as soon as I can. What a pain in the petunia's. I'd appreciate knowing about others that have encountered the same fate or does everyone just nuke the retaining clip, screw the piston in until it's flush (if that's possible) and then life is good. I'm tempted to order some new riveted pads as old ones in my collection seem to consistently have a thinner backing plate (about 1/8 inch). Two eighth inch plate pads would have easily fit. Not that a thinner backing plate is good, in fact, it may have led to warping issues and the consistently thicker plates I've seen in the samples I measured. Maybe it's a six sigma thing.

My son and I have been working on this build for a year and, like others before us, we have found the information and encouragement here to be invaluable. Thanks for that and your continued support.

3WindCoupe

iTrader: (3)

Final entry.

I installed the left side caliper/pad combo last night. The piston was noticeably deeper into the bore than I could get with the right side so I placed the pads in and measured the gap between the pads with digital calipers (sorry for all the calipers). Bingo, 1.07 " so it mounted beautifully over the new rotor (1.04") and no pad modification needed. The right side caliper must have been screwed up somehow so that I couldn't get that last 0.1 " retraction necessary. Moral of the story: If an assembly should work and doesn't then usually something fundamental is wrong. In this case I had two calipers, a newly rebuilt one and an older one where I couldn't return the piston to it's proper fully retracted position within the bore. Until I saw a properly retracted piston I had no clue. I'll return the offending caliper and look for one that has been properly assembled - perhaps it's not too late to go to NAPA as suggested earlier.