I'm getting a 1991 GTA disc posi rear end ready to replace my drum rear in my 1991 Z28. The rear was missing some of the caliper components so I bought a complete caliper/mount/rotor setup from a 1996 Z28.

I'm using the thinner caliper brackets and have had to remove the backing shim on the axle tube/housing in order to get everything lined up properly.

The only problem I still have is kind of odd - When everything is mounted up and I spin the pinion yoke, the rotor seems to wobble slightly - enough to be seen. Even with the rotors secured tight to the hub with lug nuts, as it goes around, the rotor gets very close to the caliper mount, then far away, then close. It's all probably about a 1/16", but I can see it with my eye. To be sure, everything is 100% tight so it's not a hardware issue anywhere. Both rotors appear to be doing the same exact thing; having exactly the same amount of movement/wobble as the axle shafts turn. I should mention, I replaced the axle bearings and seals as part of this freshening up of the axle.

The rotors are used, albeit not much as there is zero rust on them. They are slotted, which is why I elected to give them a try. Does this sound like rotors that are warped or something else? Is this normal and I've just never seen it up close like this? Is it possible that something is out of alignment in the rear end that is allowing both axle shafts to wobble somewhere?

Any advice is appreciated!

 

Sounds like bent axles.

Mark an axle and a rotor, at the spot where the wobble reaches its full extent one way or the other; take the rotor off and turn it 180°; (well, you can't exactly, but turn it all you can, which would be 144°) re-run your test and see if the wobble lines up with the mark on the rotor (rotor is bad) or the axle (axle is bent).

My money is on the axle.

You will need a set of these http://www.summitracing.com/parts/sag-ev10-4

 

That's kind of what I was afraid of - Would the axles in my drum rear be a direct replacement or would I need disc brake-specific axles?

 

Same thing I went through when I swapped out the rear in my 89 for a posi disc rear. The took it from a 1983 Trans Am in the junkyard and the car apparently had more damage than was visible. This is the wobble I had before installing new axles....



Disc axles are different than drum axles, rotors don't fit the mounting flange of drum axles IIRC. And count the splines on the axles you're removing, 1996 model years had 26 spline axles and the link provided by sofakingdom is for 28 spline axles which your 91 would have.

 

Both 91's would have 28-spline axles.

From the 10-Bolt FAQ:

Are Drum/Disc Axles Interchangeable?
Yes, but the drum axle flange will have to be turned down to fit the rotor.* The flange diameter on the drum shaft is 6.0"; the disc shaft is 5.850" and the ID of the brake rotor is 5.900". (Oddly, there is no distinction made between drum and disc when ordering after market axles.)

*The exception to the above seems to be the drum to PBR swap. Apparently LT1 rotors (93-97) have a 6.25" ID which will fit over the drum flange. Raybestos part number 56407.

JamesC

 

Thank you James. I read 1996 in there and assumed that was the housing he acquired instead of just the brake components...

 

Realizing I would still probably have to swap out the axle shafts anyway, I went ahead and did the rear end swap today - I didn't get 100% finished, but it did feel nice getting the old drum rear end out and getting the rear disc 3.23:1 rear end in place! Sure enough, the axles were warped and the wheels wobbled similar to the video you posted. I'll order some new ones sometime in the next couple weeks.

I also went ahead and swapped out my proportioning valve with one from a 1990 IROC-Z w/rear discs. Before the daylight was gone, I attempted to figure out how the new pads went in and wound up getting confused. I've done tons of brake jobs, but I can't figure out how the outside pad stays in and doesn't fall out!??! I tried looking up some pics or videos of the PBRs but didn't find anything. Also, I assume the pads with the little nub on the ends of the backing plates are the inside pads, right?

Sooooo... Unfortunately, even after all the work today, it is going to be a while before I can drive my Z28 again! It'll be worth it though; since swapping my T5 in a couple years ago, my gearing has been WAAAAYYY off - The T5 and 2.73 gears were great for highway cruising, but really stunk for any type of spirited driving.

 

have you tried bolting a wheel to the axle without a rotor to see if it still wobbles?
my money is on the rotors not being totally seated on the axle flange.

 

I haven't, but that's a good idea - I'll have to give it a shot this coming weekend.