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1988RedT2
1988RedT2 MegaDork
6/10/22 8:50 a.m.

So I've been meaning to get to the RX-7 brakes for a while now.  You know, brakes.  You don't really need them, but they're kinda nice to have if you need to slow down in a hurry.  The car's been fine for me to putter around in, but lately my wife and my older son have both been pestering me to drive the car.  And I want them to have brakes.

I have a new set of pads that I bought ten (?) years ago, so I ordered some new rotors and pulled it apart yesterday.  Got the new rotor on, and went to push the pistons back in the caliper to make room for the new pads.  Three of the four pistons move, one does not.  Sure enough, seized.

New calipers appear to be NLA.  I have the Mazda caliper rebuild kit and I see that Mazdatrix sells 4-piston caliper pistons for 43 bucks each.  So now I'm thinking I'll pull the thing down and rebuild it.  Not sure how extracting a seized caliper piston is going to go.  Not sure what the bores are going to look like.   Edit:  The last calipers I rebuilt were single piston.  Compressed air gets the piston out very nicely.  How does that work with a 4-piston caliper?

Do I have any other options?  Anybody care to share experiences?   Advice?

Thanks!

jgrewe
jgrewe HalfDork
6/10/22 9:40 a.m.

Keep the three that move in their spots with wood blocks and clamps etc. If air doesn't get the last one to move hook it back up to the master cylinder.  Once you get it out, hold it in place against the seal to use air to get the rest out. Its kind of a dance to get them all close to the edge so you can pull them out by hand.

The bore is less important than the seal groove and the piston finish. As long as nothing is jamming the piston movement, run them.

Brotus7
Brotus7 Dork
6/10/22 4:09 p.m.

If blocking the other pistons and applying compressed air doesn't work, another way to remove the stuck piston is to hook up a grease gun and use that to hydraulically press it out.  Check out the free Europa thread, I seem to recall him doing it there.

fidelity101
fidelity101 UberDork
6/10/22 4:12 p.m.

I'm dreading doing this reading it myself now. even rockauto is out of stock/NLA (front and rears)

 

jimbob_racing
jimbob_racing SuperDork
6/10/22 4:30 p.m.

Sounds like a perfect time to upgrade to Brembo or Wildwood units.

Somebody must have a kit out there. 

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
6/10/22 5:25 p.m.

If you are not squeamish you can drill holes in the caliper, drive the pistons out with a punch, tap the holes 1/8 NPT, and plug them.

 

Wilwoods are problematic because the Wilwood ears are really, really close to the Mazda knuckles, except for the holes that are too far to oval out and too close to make an adaptor.

The single pots are NLA too.

wvumtnbkr
wvumtnbkr PowerDork
6/10/22 6:29 p.m.

Fd calipers.  Bolt right on.  Use the same pads too.

 

If you can find some....

 

I have rebuildable cores if you need one.  Not pretty, but I'll make sure the pistons move.

imgon
imgon HalfDork
6/10/22 8:44 p.m.

+1 on the FD calipers, but even those are really hard to find.  Some of my 15" wheels just barley fit but a little filing fixed that. I have a spare set of FC's that I need to learn how to rebuild. You can get rebuild kits from NAPA as well.

1988RedT2
1988RedT2 MegaDork
6/10/22 10:02 p.m.

Thanks for the advice, guys!  I will press onward with rebuilding these.  I've got a spare one for this side that I snagged at the junkyard a decade ago if I can't make this one work.  Didn't mess with it today, as I was doing boat things, getting the old girl ready for her first outing this year.  Spent the late afternoon/early evening on the water with the family and had a nice time.  On the way out, I grabbed the saltwater tackle box by mistake.  My wife really wanted to fish, so we improvised.  She caught a decent catfish on a piece of bacon off a leftover bit of a Subway sub.  Never give up!  smiley

1988RedT2
1988RedT2 MegaDork
7/1/22 7:23 p.m.

Well, another month has bit the dust.  The RX-7 is still up on jackstands with the right front wheel off.  My son and I got the pistons out today.  Three were easy, one not so much.  First movement was further in with the benefit of a big C-clamp.  Second movement was rotational with the caliper secured in a vise and a pipe wrench carefully applied to the top of the piston.  After lots of Blaster and working it gently (but firmly) back and forth, it yielded. 

So, I think I will order two new pistons.  The three look like they will clean up fine.  The fourth is marginal, and I don't want to mess with this again.

Two questions: 

1)  Can I soak the pistons in Evapo-Rust for a day and not hurt them?

2)  I had the caliper in Purple power before pulling it apart.  Can I soak it in something to clean it up further, then rinse and blow it dry with compressed air?

 

What it looks like now:

caliper

 

David S. Wallens
David S. Wallens Editorial Director
7/1/22 7:37 p.m.

Hopefully it all goes back together well.

And because we don't make these videos for our heathsmiley, something to share:

 

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
7/1/22 8:40 p.m.

The reason they were stuck was all the corrosion in the bore between the seal and dust boot.  I see a little corrosion on the other side of the seal, too.  If the seal groove has pits, new seals may leak.

1988RedT2
1988RedT2 MegaDork
7/1/22 9:06 p.m.
Pete. (l33t FS) said:

The reason they were stuck was all the corrosion in the bore between the seal and dust boot.  I see a little corrosion on the other side of the seal, too.  If the seal groove has pits, new seals may leak.


Agreed.  What's the best way to clean up corrosion in the bore?  I'm pretty sure this caliper is going to end up being way better than the junkyard part I pulled ten years ago.  I'm still working on getting the pads out of that one.

1988RedT2
1988RedT2 MegaDork
7/2/22 11:28 a.m.

In reply to David S. Wallens :

Wait a minute!  That video is hosted by Classic Motorsports.  Are you trying to imply that my 1988 Mazda RX-7 Turbo II has somehow gotten old enough to be "old school" or even "classic?"  I remember like it was yesterday seeing a black Turbo II brand new in the showroom when I bought my 626, and that was just a few years back.  You kids sure do have a funny sense of what's "old."  Like as soon as the warranty expires, it's like "old school." cheeky

 

Edit:  Three of the four pistons cleaned up nicely in the EvapoRust.  One is well-pitted and will not be re-used.  I'm going to mow some grass and get back to the caliper, pull the seals, and see what I've got.

Edit #2:  Pulled the caliper piston seals.  The suspect groove is pretty corroded.  Anybody know if Hylomar will stand up to brake fluid?

1988RedT2
1988RedT2 MegaDork
7/12/22 4:03 p.m.

Another week gone by.  Got my new pistons from Mazdatrix a few days back.  Got the new seals in today and pushed in the pistons.  Three of four went in easily with finger pressure, but the suspect bore gave trouble.  I had test-fit the pistons in the bores before I inserted the seals, so I knew my problem was the seal groove.  I removed the seal from that bore and proceeded to gently scrape off some of the corrosion in the groove with a right-angle pick, much as my dentist does when he's scraping built-up tartar from my teeth.  Good thing for him that he doesn't have to worry about whether or not his work is good enough to prevent brake fluid from leaking past at over 1000 psi.  Clean thoroughly one more time, insert seal, insert piston.  Success!

Put new rotor on hub, bolt caliper to knuckle, what's this?  The caliper is literally contacting the rotor.  Remove little shims from caliper ears, try again.  Caliper still hitting rotor, just not as much.  Curse new rotors.  Remove new rotors, put on old rotors.  Fit no problem, but very little clearance.  Check hub flange for gunk.  Scrape gently, clean, looks good.  Try new rotors again.  Still hits.  Gah!  New rotors are like 22 mm thick.  Old rotors are 21-ish.  Min thickness is 20.6 mm.  Just not much room if parts are a little off.

I don't really want to run the old rotors.  They've rusted a bit, could well be the 35-year old original equipment.  I might try getting the Mazda ones for 90 each.  These Centric rotors were only 25 each from Rockauto.

Ain't nothin' easy!  angry

1988RedT2
1988RedT2 MegaDork
7/21/22 2:05 p.m.

Okay.  Progress update.  Mazda rotors from Mazdatrix came today.  Naturally, I had to brave the 95 degree heat and run out to the hot garage and see if they fit.

calip

 

And they do!  But just barely.  First time I've encountered aftermarket rotors for a specific application that DID NOT FIT.  As you can see, the Mazda rotor to caliper clearance is minimal, but it does not drag.

Also, Mazda has deleted the rotor to hub screws on this iteration of the part.

Onward!

1988RedT2
1988RedT2 MegaDork
7/22/22 4:25 p.m.

Okay, Houston, we still have a problem.  Couldn't be sure it was dragging with the wheel in the air, but on the ground and turning, it definitely drags.

So that leaves possible cause as wheel bearing?  I'm having trouble believing there's that much play there.  I'm about to jack it up again and give it a good shake.  Bearings have maybe 2000 miles on them from when I replaced them 15 years ago. 

Logically, wouldn't the rotor be centered in the gap in the caliper?

What else affects rotor to caliper clearance?  Can my spindle be borked?

 

sevenracer
sevenracer HalfDork
7/22/22 4:49 p.m.

I think it would work perfectly if it just had rotor to hub screws! - Sorry, couldn't resist.

I think you mean that the disk is dragging against the caliper bracket - right? Not the pads dragging too much on the disk?

If so, I would double check that the center bore of the rotor is large enough and chamfered on the back (if needed) to sit flush to the hub. Is the other side more centered? I doubt the spindle is the issue.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
7/22/22 5:34 p.m.

How are you sure that they are dragging?

wvumtnbkr
wvumtnbkr PowerDork
7/22/22 5:42 p.m.

I've used any old rotors on my 2nd gen and never had this issue.  It seems like the rotor is sitting a bit proud of the hub, or the hub is not seated or something.

 

I've run centric, raybestos, mazda, etc and have never had that issue.  Something is wonky.

1988RedT2
1988RedT2 MegaDork
7/22/22 5:43 p.m.
Pete. (l33t FS) said:

How are you sure that they are dragging?

Check the picture above.  The clearance on the inboard side is generous--at least 1/4".  I drove the car around my cul-de-sac and it was quiet when going straight, but on a turn, there was a distinct grinding noise.

1988RedT2
1988RedT2 MegaDork
7/22/22 5:46 p.m.
sevenracer said:

I think it would work perfectly if it just had rotor to hub screws! - Sorry, couldn't resist.

I think you mean that the disk is dragging against the caliper bracket - right? Not the pads dragging too much on the disk?

If so, I would double check that the center bore of the rotor is large enough and chamfered on the back (if needed) to sit flush to the hub. Is the other side more centered? I doubt the spindle is the issue.

Not the pads, it's the caliper body that's dragging on the rotor.  Clearance is nil on the outboard side, inboard side clears by 1/4" or more.

I've cleaned up the hub flange.  Rotor sits flush.

I'm about to go back out and check the other side.

1988RedT2
1988RedT2 MegaDork
7/22/22 6:33 p.m.

So the other thing that's bugging me is that this is the same side that showed damage on the front subframe, my adventure therewith documented herein:

https://grassrootsmotorsports.com/forum/grm/operation-born-again-rx-7/174578/page3/

Trying to understand how this might be relevant.

amg_rx7 (Forum Supporter)
amg_rx7 (Forum Supporter) SuperDork
7/22/22 7:30 p.m.

Is there some kind of shim type thing that is supposed to be installed between the caliper and upright?

1988RedT2
1988RedT2 MegaDork
7/22/22 8:09 p.m.
amg_rx7 (Forum Supporter) said:

Is there some kind of shim type thing that is supposed to be installed between the caliper and upright?

Actually there are.  Each ear of the caliper wears a thin steel shim/clip/thingie that my Haynes manual calls a "guard plate".  They would seem to have the effect of locating the caliper inboard a small fraction of an inch.  If I remove them, I might have enough clearance.  Why are they there in the first place?

 

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