I am going to convert my 9-bolt borg warner rear out of an 86 trans-am to the newer 12" rotors. I was planning on buying the backing plates from David Auster .
I emailed gmpartsdirect.com and they said that the part# mattered on whether I had a 7.75" or 7.625" ring gear. Why would this matter? How do I tell which one I have? Where do I measure from? Please help me. I want to make sure that I get the correct backing plates.

 

A 9-bolt is a 7.75". It's the size of the ring gear.
The newer 10 bolts (somewhere around '90(????)) are 7.625". Earlier ones were 7.5".

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'89 Formula - Thunderchicken - 355 Vette L98/T-5/3.45 9-bolt/black/t-tops - the fun car!
'90 GTA - The GTA - 5.0 LB9/TH700/3.27 9-bolt/black/t-tops/tan leather - the cruiser.

 

Right bracket for '89 9 bolt with PBR aluminum calipers is PN 10136851. That's available. Poughkeepsie Chevrolet here in NY has 2 on the shelf. $44.
Left rear is pn 10136852... supposedly discontinued. I'm looking for one myself right now.

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'89 Formula - Thunderchicken - 355 Vette L98/T-5/3.45 9-bolt/black/t-tops - the fun car!
'90 GTA - The GTA - 5.0 LB9/TH700/3.27 9-bolt/black/t-tops/tan leather - the cruiser.

 

Caliper carrier #10112652 need 2 $23.22 ea.
Backing Plate #10136853 L/H $10.73.
Backing Plate #10136854 R/H $10.73.
From gmpartsdirect.


[This message has been edited by Tad (edited May 31, 2001).]

 

Is that for 9 bolt stuff or 10 bolt???? I was just looking at the stuff at lunchtime.
'89 9-bolts were the only ones that came with PBR calipers. Earlier 9 and 10 bolts had the crappy GM rear disks. '90 and up 10 bolts had PBR's also.

Personally, I swapped the 10 bolt in my GTA for an '87 9 bolt and am trying to ditch the old GM style rear disks that came with it.

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'89 Formula - Thunderchicken - 355 Vette L98/T-5/3.45 9-bolt/black/t-tops - the fun car!
'90 GTA - The GTA - 5.0 LB9/TH700/3.27 9-bolt/black/t-tops/tan leather - the cruiser.

 

It's the stuff for the PBR calipers. They will bolt on 10 bolt. I put them on 87 10 bolt out of T/A. They sould bolt up to a 9bolt but not sure.

[This message has been edited by Tad (edited May 31, 2001).]