85 Rx7 - rear rotors and fluid questions

[G-Man]

More newbie questions! IT-A 1985 Rx-7, 4.77 Kia Limited Slip.
What's the recommended/preferred differential lube?
For the tranny, any reason not to use Mobil1 75W-90 LS (synthetic GL-5)?
And, my rear rotors are warped - the pad oscillates 1/8" idling in fifth gear (wheels off, car safely on stands). Sigh. I need new rotors, right? And they're a bear to change, right? Any tips?
Thoughts on cooling rear brakes?
Thanks for the continuing help!

[TomL]

If your pads/rotors are moving back and forth that much, it's more likely to mean that your discs aren't firmly attached to the axle. They are normally held in place by a couple of screws, but the screws tend to disappear over time. This isn't a problem, since the discs are held in place by the lug nuts/bolts when the wheels are on. However, when the wheels are off the discs flop around. Check and see if you still have the screws holding the discs in place. If not, I'll bet you can move the discs side-to-side by hand, and that's what you're seeing. Either replace the screws or temporarily screw on a couple lug nuts to hold the discs solid and the wobble will probably go away. If not, then you may actually have warped discs. But I've never seen it (in the rear) in the dozen years I've been running RX7s.

Changing the rear discs is very simple. The only possible problem is that the disc attaching screws can get rusted and cause you to strip the the center out of the phillips screw heads when you attempt to remove them(that's why they tend to disappear :D ). If they do, a vise grip on the edge of the screw head will usually work. If not, then you drill them out. Then you disconnect the caliper (one bolt) and remove the brake bracket (two bolts) and the disc comes right off.

I've never heard of anyone having rear brake cooling issues with an IT RX-7. I've never seen anything done beyond removal of the backing plates.

I've always used Mobil1 75/90 gear lube in my car (trans and rear axle). Works fine.

[dickita15]

Tom is right on as far as he goes. Most likely the rotor is moving on the axel, however if the rotor is really running untrue it could very likely be a bent axel.

[G-Man]

Thanks for the help and info.
I took another look at the rear axle and made a video. You can see the pad oscillate on left rotor and the swept area seems off-center - is the rotor perpendicular? On the right, less pad oscillation but some wiggle to the rotor?
All suggestions welcomed - I'm hoping it's nothing!

Direct URL:youtube.com/watch?v=7BGd_88GOx0
http://www.youtube.com/watch?v=7BGd_88GOx0

[lateapex911]

Weird.
I'd pull the rotors off and lay them on a flat piece of thick glass. They should be flat. If not toss them.
And I'd get a runout gauge and I'd check the axle. I'd check end runout and I'd also see if I could find any other slop of bend to it.

Sometimes guys try to get camber in these. (Like it's 'worn") It's a 'trick' because there is no camber spec listed in the manual, so it can't be 'cheating' if it the amount it's supposed to be is non existant. Of course, if they don't know what they're doing....things can get pretty wonky. I'd check the housing as well. Reference it to a perfectly flat surface and check the faces for square in both directions (up/down, forward/back). I doubt this is the issue, but, while you're there....

Regarding the screws. If you have them they are often rusted. Shoot some PB Blaster and let soak. A little heat will help. Then, use an Impact Screw driver. (You mount a bit in it ...a Phillips in this case...then strike the back of it with a hammer. The mechanism turns the force into rotation, and sets the bit deep, avoiding cam out all at once. Pretty neat device and cheap.

Often the discs refuse to come off the axle flange as well. This is another time for PB Blaster and a hammer.

[dickita15]

No offence Jake but do not get distracted by the cambered rear end thing. That is done by bending the housing a bit but the axels still have to be straight. Pull the rotors as Jake said and check for true. You can also swap them side to side and see if the problem follows the rotor. As Jake said next is to check the axel. I ended up bringing a bunch of axels to a machine shop and they checked them for runout. Half of the ones I had kicking around were no good.

[Dano77]

From looking at the video it appears that the rotors have a ridge on the inside. Kinda like a record player. That causes the pads to follow the groves similar to the needle on the record.
Rotors are cheap at Rockauto.com and your local Advance Auto Parts store.

Start there and see if the pads walk around. they will move some as the anti rattle clips are missing on that model. The padal Pulsating under your foot is what you need to worry about. As long as the rotor surface is flat and dosent have any groves or ridges in it,the pads will find a place to live.

Dan

[TomL]

I agree, that's a weird looking video, although I think the off-center wear pattern in the inside of the left disc makes the wobble appear worse than it actually is. As Dick has said, I'd check the runout on the axle flange (with the disc off) as the first thing to do. If the flange is bent, nothing you do to the brake components is going to help. It's not likely, but it has happened. Assuming the flange is straight, then measure the runout of the disc itself with it back on the axle. If runout now appears, then the either disc itself is not true, or possibly a piece of junk has gotten between the disc hat and axle flange causing the disc to not run true. I'm inclined to agree with Dano that your disc appears to have a significant ridge on the inside and that getting a new set might be a good idea, regardless. A new set will last for years, so it's not a big investment.

[G-Man]

Thanks for the help and suggestions! I'll work on it next week and post what I find out.

[lateapex911]

No offence Jake but do not get distracted by the cambered rear end thing. That is done by bending the housing a bit but the axels still have to be straight. Pull the rotors as Jake said and check for true. You can also swap them side to side and see if the problem follows the rotor. As Jake said next is to check the axel. I ended up bringing a bunch of axels to a machine shop and they checked them for runout. Half of the ones I had kicking around were no good.
Dick, understood on the bending of the housing, BUT, if an idiot (not that there are any of THOSE out there, LOL) did it wrong, and used the actual axle flanges as bending points instead just the housing, it could get messy....

I agree that it is an unlikely situation, esp in this case.