47-55.1 Brake rotor conundrum

[51 3600]

Who knew that buying rear brake rotors could to be a major headache!
Without going into the long background story I'll start with what I was told many months ago. The rear axle on my '51 3600 is "from a '02 Z28 Camaro...well really a Transam". So I ordered a pair of rotors from Rock Auto and found they didn't fit. Rotor was jam up against the backing plate, disc was too thick to fit in the caliper and wheel lugs barely protruded through the rotor.



Checked with person in the know about the axle. "On second thought, I think instead we mounted a 2000 Blazer 4WD drive axle." Ok. Ordered a pair of Blazer rotors from RA. They didn't fit either. Disc fit in caliper but rotor was too deep just like the Camaro rotors and tight up against the backing plate. "Maybe a RWD?" Got smart this time and went to Auto Zone and bought one rotor to try. Doesn't fit. Disc thickness ok but still jammed up against backing plate and rotor about 1/4" away from the axle flange while still up against the backing plate.



So now I've spent a bunch of hours on the internet trying to figure out what I've got. Not being a mechanic doesn't help interpreting a lot of what I researched. A little of what I've learned:

Diff is Borg Warner 9 bolt used on 3rd gen F bodies. 3.27 gear ratio.



Calipers came with this axle and seem to fit fine but don't look like Camaro calipers at all and are similar but bigger than Blazer caliper pics I can find. One comment I found said Blazer 4WD calipers mount forward of axle and RWD ones mount rearward.


Other misc. info:
Backing plate to backing plate dimension of axle is 57.25"
Axle flange to axle flange dimension is 61.75" which would make wms to wms approximately 62.25".
Camaro rotor measures 2.375" from back surface of disc to inside of rotor wms surface. That dimension can't be bigger than 1.75" in order to fit on my axle and work with the caliper.

Sure could use some help from the wise counsel found on this forum. What have I got? Sorry this is so long. As Mark Twain said "If I had more time, I could make this shorter."

[_Ogre]

.Diff is Borg Warner 9 bolt used on 3rd gen F bodies. 3.27 gear ratio.

58 TRUK has an 86 3rd gen trans am 9 bolt rear axle under it. My understanding is that only the trans am got the Borg Warner 9 bolt. Mine had the 3.27 tag on it, best I can figure is it's a 3.73 ratio. Your caliper looks nothing like what is under TRUK. 3rd gen rear discs have the parking brake cam gizmo. I'm traveling and don't have access to pictures, look at my build thread to see if anything jogs your memory. Look at 3rd gen trans am in the forums

[dsraven]

maybe take pics of those tags and talk to a gm dealer or a driveline parts store. they may have info based on that as might an F body forum. some posted up pics of the set up from different angles may also tweak an F body forum member's memory as well.nthe last F body diff I used had those lever style park brakes as well, like a cadillac had for a few years. maybe a walk thru at the pick n pull may help or an F body specialty wrecker.

[dsraven]

I guess the other optiin is to auto zone every rotor available for a 3rd gen F body until you find a match.
good idea to start a log that documents part numbers

[51 3600]

Thanks Ogre and dsraven for your ideas and suggestions. Spent a good 3 hours on the internet searching and educating myself further. More thanks to thirdgen.org which led me to gmpartswiki.com (and back to thirdgen to interpret what I found there) I specifically know what I've got. It is a BW 9 bolt 1988 F with 3.27 GR (matches my spinning the rear wheel method) and 7.75 RG with disc brakes, positraction and performance package used on '88-'89 Camaros and Firebirds. This baby is rare which equates to hard to find and expensive. 144 vehicles so equipped produced out of 301,335.

Good news is rotors look to be available. From sites like Rock Auto, the specs match with dimensions I've determined are needed. Local parts stores don't have them in stock so ordering online....again.

Thanks again gentlemen!

[dsraven]

forward progress. awesome.
if it were me, and I won the lottery, that axle would be on the classified list and a newer more common axle would reside under there, haha. when I need brakes, an axle seal or bearing when on a road trip, for some stupid reason, I want the local parts guy to have them in stock at closing time.

[mr48chev]

Some searching shows that the 9 bolt 7.75 rear axle showed up in the late 80s. The guy who gave you the info may be a out 15 years off.

[51 3600]

Originally Posted by dsraven

forward progress. awesome.
if it were me, and I won the lottery, that axle would be on the classified list and a newer more common axle would reside under there, haha. when I need brakes, an axle seal or bearing when on a road trip, for some stupid reason, I want the local parts guy to have them in stock at closing time.

Good point especially as I'm in the lottery-winning-if category. However, in my excitement of making a rare find, I succumbed to the contemporary tendency to misrepresent the data. I made it sound like there were only 144 9 bolts used in the total production of 300K+ vehicles. Not true. (Besides, logic tells me GM wouldn't buy just 144 diffs for a production vehicle. No economy of scale there.) Actually of all the 9 bolts used in '88 and '89 on Camaros and Firebirds only 144 had the "performance package", RPO 1LE along with other options. I don't know what the performance package is.

Originally Posted by mr48chev

Some searching shows that the 9 bolt 7.75 rear axle showed up in the late 80s. The guy who gave you the info may be a out 15 years off.

That's what I found too. Sometimes I think he's "off" more than that, ha. But he's a good guy, just has too much going on sometimes.

[1project2many]

Part no 10079888 can be found here:
http://www.gmpartswiki.com/getpage?pageid=37153
1989 F car, W/ single piston caliper, W/O 1LE brakes

Duralast 55022 rotor. Pictures and dimensions below.
Calipers 18-B4346 and 18-B4345. Picture below.
Pads should be easy to find.

[1project2many]

There is an upgrade path:
http://www.ws6transam.org/1LEbrake.html
Rear brakes are about 3/4 down the page.

[51 3600]

Thanks 1p2m. Rotors I ordered through Rock Auto exactly like the ones in pics you attached. Also appreciate the great brake upgrade info. (Don't know how people find all this info.) I'll save it but pass on the upgrade for now as I will be driving my AD as a old truck and not a TransAM, ha.

[my56chevytruck]

Originally Posted by 51 3600

Who knew that buying rear brake rotors could to be a major headache!
Without going into the long background story I'll start with what I was told many months ago. The rear axle on my '51 3600 is "from a '02 Z28 Camaro...well really a Transam". So I ordered a pair of rotors from Rock Auto and found they didn't fit. Rotor was jam up against the backing plate, disc was too thick to fit in the caliper and wheel lugs barely protruded through the rotor.

Attachment 2162873

Checked with person in the know about the axle. "On second thought, I think instead we mounted a 2000 Blazer 4WD drive axle." Ok. Ordered a pair of Blazer rotors from RA. They didn't fit either. Disc fit in caliper but rotor was too deep just like the Camaro rotors and tight up against the backing plate. "Maybe a RWD?" Got smart this time and went to Auto Zone and bought one rotor to try. Doesn't fit. Disc thickness ok but still jammed up against backing plate and rotor about 1/4" away from the axle flange while still up against the backing plate.

Attachment 2162874

So now I've spent a bunch of hours on the internet trying to figure out what I've got. Not being a mechanic doesn't help interpreting a lot of what I researched. A little of what I've learned:

Diff is Borg Warner 9 bolt used on 3rd gen F bodies. 3.27 gear ratio.Attachment 2162875

Attachment 2162876

Calipers came with this axle and seem to fit fine but don't look like Camaro calipers at all and are similar but bigger than Blazer caliper pics I can find. One comment I found said Blazer 4WD calipers mount forward of axle and RWD ones mount rearward.
Attachment 2162878

Other misc. info:
Backing plate to backing plate dimension of axle is 57.25"
Axle flange to axle flange dimension is 61.75" which would make wms to wms approximately 62.25".
Camaro rotor measures 2.375" from back surface of disc to inside of rotor wms surface. That dimension can't be bigger than 1.75" in order to fit on my axle and work with the caliper.

Sure could use some help from the wise counsel found on this forum. What have I got? Sorry this is so long. As Mark Twain said "If I had more time, I could make this shorter."

very good detective work on your rearend mystery. I'm sure you'll be future help for someone asking about "what rear axle do I have".