I have a 1970 Oldsmobile Cutlass with a Type "O" rear diff. Thanks to Brian Trick, I was able to install a 3.23 posi unit. My next upgrade is to put on Wilwood disc brakes. They want to know which axle flange I have. Can anyone help me identify which of these diagrams corresponds to the Type "O"? I want to think it is the center one . . . ?

 

The left is marked Chevy 8.2-8.875 10/12 bolt which is a C clip axle type and the round one on the right for Pont/Olds is referring to the 57-64 big car 9.3 axle but the middle one id marked BOP is for a 8.2 BOP 10 bolt with bolt in axles.

The 4 bolt backing plate/ axle retension plate pattern of the square ones left and middle is the same pattern as yours 12 bolt Type O (8.5 10 bolt) the bearing ID shouldnt matter if your using the same axles you have. The caliper bracket is going to use the 4 bolt pattern

 

Thank you but based on your reply, I still don't know which of the diagrams is the Type "O"? Or are you saying it's none of them?

 

Yep none of the above but .. the disc brake stuff should only be concerned with bolting to the 4 bolt pattern that is the same on all of the chevy and bop 10 bolt 12 bolt A body and F body stuff like in the left and middle pics. Not the round on on right. If you are reusing original axles and bearings and the rotor merely slips over the stock axle studs and flange the caliper bracket with bolt on and line up as designed regardless of axle bearing should not be an issue unless I am missing something. The Type O is not truly BOP since it was O only , so that one above is obviously BOP 8.2 10 bolt

 

I guess the most important thing is the disclaimer that says, "The direct bolt-on kit is ready for installation on your 10-bolt, non C-Clip differential with 2.75" bearings." Has something to do with the "offset".

 

Yep just measure the offset on yours, it should be the same the 68-72 A body had the same rear width across the board even with 5 different types (12 bolt Chev, 12 bolt type O, 10 bolt corp 8.5, 8.2 10 bolt chev, 8.2 10 bolt bop) and all the brake drums are same as are the backing plates swap on 64-72 A body and 67-690 and 70-81 F body

 

how come they dont give you the offset measurement on their drawing though?

 

Thanks for all your help!

 

Not quite. The backing plate on the early O-Type has a larger hole for the shaft/bearing to fit through. I discovered this when I put a '68 O-Type on my car and the axle bearing wouldn't fit through the hole in the '71 10 bolt backing plates. The flanges do have the same bolt pattern, though.

Early O-Type axle bearing = RW507C, bearing diameter = 2.7475"

'70-up O-Type/8.5" 10 bolt axle bearing = Set9, bearing diameter = 2.5591"

So from the diagram the 2.75" flange has a large enough hole to fit the early rears.

 

Right, I figured the Type O bearings were bigger OD as I looked them up on Rock Auto but the offset horizontally of the axle flange mounting surface to the 4 bolt flange bracket should be a standard dimension and I cant' think why any of the bearing stuff would matter for a caliper bracket not to work same as the rotor should slide on no problem. The 70 Type 0 is a different bearing than the 67-69 type O though so being a 70 type O has axle and bearing interchange with the 71-72 Corp 8.5 Cutlass rear in other thread about some interchange of 8.5 side gears and mini spools in type O diff but using the 8.5 axles because of two different types of 28 spline a 60 degree and a 45 degree.

 

Yeah, I was just pointing out that there was a difference between the backing plates, but it isn't one that matters in this situation.

 

Resurrecting this thread because I have an opportunity to buy used what is likely the same Wilwood kit for the BOP 8.5” with the bigger 2.75” axle bearing (140-13511). My axle is the later 2.56” axle bearing. I’m trying to determine if the two types of axle/tube/flange (stock) are dimensionally different in any ways

other than the OD of the axle bearing. If not, I feel like I should just be able to modify the bearing retaining plate (ref, fig 2 in drawing) and the kit should install correctly.

I contacted Wilwood about it and their response was simply that they don’t sell a kit for the 2.56” bearing.

The $$$ savings would make buying the used kit very appealing if I could do it correctly. I’m not a machinist so would just be using my little mig and hand tools, but as long as the plates aren’t cast with tricky reliefs, I feel like I could do it.

Does anybody have experience with this kit and could offer an opinion? Am I oversimplifying it or missing something important?

 

When I converted my O axle to disc, the vendor was no help, as they assumed the O axle bolt pattern required a different retaining plate.

I figured, as you did, that I could make a new retaining plate. But I found the O axle bolt pattern was the same as on a Chev 12-bolt axle. I used the supplied retaining plate for the 12-bolt.