Centering fixed rear calipers

[newschool72]

I have fixed 4 piston calipers on the back of my 72 Camaro with Hotchkis TVS springs and the factory 10 bolt 8.5 rear diff. I have been trying to reduce the amount of axle end play to try and avoid the dreaded piston bump back syndrome . In my stock style eaton posi unit, the clutches have a fair amount of wear so I had .045 right and .055 left side end play. Not being able to afford a complete rebuild on the rear right now, I came up with a way to get the end play to the ideal .005 to .008 . I feel good that I shouldn't have too much issue with bump back now.
My question is, how important is it to have the caliper centered on the disc. I am going to shim the calipers to get them centered anyway, but just wanted to know how important is it for a good pedal and to minimize brake drag. Will the pistons just "self adjust" and the side with more clearance just ride with more piston exposed, or does it matter how much piston is exposed as to how quickly the piston reacts to brake pedal movement.
I already know I will reduce brake drag significantly on the front by centering the front calipers, because the outer pistons are all the way in and dragging the slight warp in the front disc's. The calipers in my particular setup are very close to the fresh rotors on both sides , so there isn't much wiggle room on side to side orientation .
While I have your attention, is there any special attention that needs to be had when turning dics's that are drilled and slotted. I will most likely go ahead and have the fronts turned to get the slight warp out of them. Thanks in advance for the input.

[Bryce]

The calipers should be spot on centered. At least that is what wilwood has in their instructions.

What calipers are you using?

[Apogee]

In my experience, the calipers location relative to the rotor doesn't impact performance, however it does increase the likelihood of caliper to rotor interference the further away from center you are. The pistons will adjust side to side, just like they do for (un)even pad wear and, so no worries there. Most fixed calipers specify a recommended tolerance range for mounting guidelines, for the C6 Z06 calipers it is +/-.02" [+/-.5mm], but this is generally more a function of the width of the slot in the caliper for the rotor relative to the rotor thickness than anything else.

When turning drilled and/or slotted rotors, take very light passes since the discontinuous cuts will otherwise be hard on the tooling and can result in chatter or an otherwise less than ideal surface finish.

Tobin
KORE3

[newschool72]

Thanks for the response , guys. I am running ABS (brand, not abs brakes ) 4 piston fixed calipers on all four corners. I just got home from getting one rotor turned. The discs have less than 300 miles on them, but the pass side front was warped as it came from ABS. What I didn't realize when I put it all together was, the warped rotor along with the caliper being set to the inside of center was making the out board pistons bottom out when the warped section of rotor would come around. I think I found the reason my car felt like it was holding back a little, LOL. That had to be causing a tremendous amount of brake drag ! It was like anti lock brakes being lightly engaged all the time on the right front.
I am going to make sure all the calipers are centered and go do some testing .Around a few turns to see if the bump back is no longer an issue and the drag strip to see how much the home made anti-lock setup on the right front was slowing acceleration.

[bovey]

I had both issues.

First: off centre callipers - had my brackets machines so they were perfect. Took time, but it was easy.

2nd: GM rear-end slop pushing pads back. Like you, I can do a full rear-end. I did a band-aid fix and installed a 10lb residual valve in and it took 98% of the issue away with out dragging. You can read about it here in detail:

https://www.pro-touring.com/threads/101080-Soft-Pedal-vs-Axle-Runout-solutions-Willwoods-(fixed-callipers)-on-a-12-bolt-GM

[newschool72]

I read the thread , bovey. Very interesting ! My car wont see the use your truck does, for a while anyway, but glad to see it made that big a difference for a simple fix. I put on my McGuyver hat for my bandaid, but I think it will work for what I will be doing ( street cruising, limited auto-X "maybe", an occasional trip to the drag strip ) I put the car together as an older school car to hang with the new pony cars by the big 3. Good braking, handling , a few creature comforts like AC, PS, PB and enough power to dip into the 11s in street trim ( w drag radials ).
I measured the end play on my axles at .045 right and .055 left. The reason is the clutch packs in the factory posi are worn. The more they wear , the more slop the axles see. I don't have the funds to rebuild the posi right now so here is what I did. After measuring the end play, I pulled the axles, drilled and tapped a 1/4 20 hole in each axle tip. I installed a 1/4 X20 bolt and ground it to 0 end play (.045 and .055 of the bolt head left ). I installed the axles and measured . Well I couldn't measure because a feeler wouldn't fit, LOL. I filed a couple thousandths off at a time, fitting, measuring about 12 times, until I had between .005-.008 end play. I don't see where there could be any complications or concerns with the way I eliminated my end play issues. In time , Im sure the bolts will wear and the end play will slowly come back. Hopefully, by then, I will be ready to rebuild the posi with new clutches.
Thanks for the input, guys.

[bovey]

That is an iterating fix newschool72 - please let us know how it works out. FYI - it only takes a few thousandths to cause pad knock back - my end play was in spec and I had huge amounts of varying sponge in the petal. Anyway, good luck. Trust me - autoX it. Crazy, crazy fun.