CRG Discussion Forum

Model Specific Discussions => Trans-Am Camaros => Topic started by: 369 on September 24, 2017, 02:29:58 PM

Title: JL8/J56 or HD Service rear end setup question
Post by: 369 on September 24, 2017, 02:29:58 PM
Hello all......I'm putting together a disc brake rear end with the "fixed mount" J56 corvette style calipers. I'm shooting for .005" axle end play......I know this will require custom c-clip thickness which will change as the posi clutches wear..I'm trying to get an idea of how often this will need to be adjusted or reset on a road car. I figured if anyone would know it would be someone here. Any guesses?

-So from what I've read, Penske would change his rear end every race so no issue here...

-the lesser funded guys...How often was this checked/changed? Was the pad knock back tolerable enough to make it thru a season?

-what about the JL8 owners.....I can't see this being a "dealer service" adjustment every other oil change or anything right?

Ok thanks guys.....Tyler
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: Jon Mello on September 27, 2017, 01:09:08 AM
No idea, really. Probably not a lot of miles put on the car and not a lot of heavy abuse so you probably could go a couple of years with no problem. Just a guess.
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: 69Z28-RS on September 27, 2017, 01:55:40 AM
I've a good friend who ran a service rear in his '69 Z28 in SCCA and later IMSA races.   He's told me that he had the rear end gears changed very often, before essentially every race, depending on the track and the speeds anticipated.  I don't recall him saying anything about having to adjust for brake wear...  I'd think there would be sufficient pad thickness to not be anything close to 0.005" tolerance... The pressure on the sides of the rotor should be essentially equal even if one pad was wearing faster than the other.
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: firstgenaddict on October 13, 2017, 07:02:02 PM
With the fixed mount calipers yes there is plenty of tolerance for axle end play, if you are worried about thickness of the C clip then set up every thing with rotor and caliper sans pads, insert different thickness C clips and check where the rotor and caliper are in relation to one another... I think you are going to have .375+ inboard and outboard of the rotor.
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: 369 on October 21, 2017, 03:25:29 PM
10-4.....thanks for the input. Im starting the assembly here this weekend so we'll see how it goes!
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: 69Z28-RS on October 22, 2017, 02:18:38 AM
take some photos, 368...  help to educate us... :)
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: 369 on October 25, 2017, 04:41:03 PM
Yea-I'll shoot some this weekend.

So....interesting tidbit here...I downloaded and printed the "chassis preparation manual" that jon mello posted up here (thanks jon btw)......they list the axle end play spec as ZERO to .015".......ZERO!....playin' with fire there once that whole deal heats up to racing temp.

I know..I know........way over analyzing  but I just thought that was kinda cool
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: 369 on January 01, 2018, 08:00:15 AM
So this thing is still moving forward....plenty of help from my buddy who did most of the welding and grinding but here is where I'm at.....everything lined up without having to make spacers like the "Untold Secrets" book talks about.....And the caliper bracket to rotor surface measurement is right in spec too!! Working on the parking brake cable mounting now.

Here are some pics
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: 369 on January 01, 2018, 03:43:37 PM
Here ya go.....it's going on a 67 so no staggered shocks.......I used a RH and LH assembly instead of the two RH deal everyone is used to seeing
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: 369 on January 01, 2018, 04:34:59 PM
So no one I talked to had a definitive answer for that endplay so we settled on a touch over zero. It's what the competition manual suggested and the posi I have is shimmed tight so it should wear in a touch more....the calipers sit square over the rotors so I guess we'll see how it flys (or stops I guess).

--sending c-clips to get hardened

--the book talks about shot peening axles and studs....I used Moser stuff....it's pretty beefy so I'm hoping it'll hang in there. The car won't see any 4 hr endurance contests so Im thinking it's good.

--Moser axle flanges are about 1/8" thicker than the 12 bolt stuff which threw some curve balls at the JL8/J56 parking brake setup....it clears, but it's CLOSE!.......I may take a little off the parking brake shoes to help this.


So yea! That's where this thing is at............HAPPY NEW YEAR EVERYONE!!!
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: Jon Mello on January 01, 2018, 11:38:44 PM
Looks great. I like how you chose to position the calipers. Thanks for the update and posting the photos. Happy New Year!
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: Trans m on January 02, 2018, 02:46:23 AM
Yes, thanks again for posting this info. I may be doing something like this someday and might look to you for some guidance. :D
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: 369 on January 02, 2018, 05:35:46 AM
Thanks.....yes sir ask away...I don't know everything about it by a long shot but I'd be happy to share what I've learned...if anyone else has an idea or tip, throw it in here too!!!
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: 369 on January 28, 2018, 01:29:38 AM
Ok so I am looking in Wayne Guin's book and it seems that original JL8 rears were mounted with one U-bolt and T-bolts on the other side of the spring pad.......As for the HD service rear/racing version does anyone know how they were mounted? Basically I was going to copy the JL8 style mount for ease or brake line routing at this point. Did you trans am boys double up on the U-bolts or did the factory stuff hang in there just fine?
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: Jon Mello on January 28, 2018, 06:28:20 AM
Most Trans-Am cars run two U-bolts on each side and don't use the T-bolts.
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: zeohsix on February 14, 2018, 11:26:30 PM
Im seeing this kind of late but have expireince in JL-8 set ups....excessive Axel play will cause piston knock back and a spongy brake pedal. Toms differentials in Idaho had different thickness C-clips for 12 boot axels. You could also have Toms run is a set of posi clutches to minimize later wear....Penske Trans Am cars used the 22 plate posi clutch packs Toms at one time had these too....the 22 plate packs arent as grainy on the clutch plate surfaces as the 18plate clutch packs are....much smoother. Breaking in the posi clutch pack is done by running on a stationary test fixture or running vehicle in on by doing figure 8s in a parking lot. The intial clutch pack wear is considerable can cause a 0.010-0.040 additional slack in C clip clearances.....the material is really hard and abrasive in nature too coming off the clutch pack plates.....NASTY stuff. I also was told by a good friend who was a mechanic way back when that a SF Bay Area Chevy dealer once warranty swapped a drum braked 12 bolt into a JL-8 Camaro as a solution to the spongy brake pedal syndrome in a street driven Z/28....way back when. 0.005 should be considered a max figure in axle endplay when building a JL/8 12 bolt with a broken in posi clutch pack. After initial break in the clutch pack wear is substantially reduced. I hope you know not to run the daisy petal type clutch socks as they can crack and fail in use...GM went to these in the early 70s to stop warranty complaints of differential chatter in the solid plate clutch packs. Im not a fan of the O ring style caliper piston seals but if you have end play and soft pedal problems they might assist in reducing aeration problems but again Im no fan of O ting caliper Pistons.
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: Jon Mello on February 16, 2018, 04:56:54 AM
Thanks for the info.  It's never too late to add to one of the threads here.
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: firstgenaddict on July 07, 2018, 07:39:34 PM
Didn't the penske cars just pack the posi clutches as tight as possible so they essentially had a spool?
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: 369 on July 24, 2018, 05:46:46 PM
I seem to remember reading in Guinns book that Penske had axle failures when that was tried.
Title: Re: JL8/J56 or HD Service rear end setup question
Post by: firstgenaddict on September 08, 2018, 10:55:14 AM
Iconel J56 Pads...
https://www.ebay.com/itm/GM-66-67-68-69-70-71-Corvette-J56-Brake-Pads-Inconel-Alloy-427-454-L88-ZR2-ZR1/112856316996?hash=item1a46c2bc44:g:iF4AAOSwjylaobEl

Another set of J56 pads... Iconel?
https://www.ebay.com/itm/66-67-68-69-70-71-72-Corvette-J56-Brake-Pads-427-454-L88-ZR2-ZR1/112856325868?hash=item1a46c2deec:g:AfoAAOSw-fNaobMl