The rear in my car just went (10 bolt 4.10 posi) and I just found one for it in the junkyard (3.73 posi) but it has rear discs. I want to keep the drums on the back of my car though. My question is can I put this disc rear in my car and swap it over to drum brakes? Do I need to make any modifications? And what is better, the 9 or 10 bolt rear? Also, if I decided to swap my car over to rear disc, what parts would I have to change on my car?

 

You can go from drum to disc by modifying the axle flange but not disc to drum. Others may disagree, but in the long run the 10-bolt, though a bit weaker, is the best choice since parts aren't readily available for the 9-bolt. Why not simply swap to discs (I'd jump on that 3.73/rear disc in a second)? All you'll require, other than the rear itself, is a proportioning valve and e-brake cables. With the rear you mention, if you so desire, you can easily swap on bigger brakes. Check the following link for more info about that:

https://www.thirdgen.org/forums/faq-...iscs-89-a.html

JamesC

 

So you cant go from discs to drums?

 

No (edited above, my bad).

JamesC

 

Why would anyone want to go from disc brakes to drum brakes

 

I just wanted to keep my drums on the car so that I wouldn't have to convert everything else over to the disc brake set-up. Why can't disc rears be converted to take drums? Isn't there someway you could get the drum backing plate on it?

 

What do you mean by convert everything else???? Just get a disc/disc prop valve. Much easier than any method that would have to be done, if it could be done, to keep your drums

 

I've been wanting to convert my drum rear over to disc brakes and put in an Eaton posi unit too. I haven't gotten the the drum rear out of a different donor car yet, but I will be removing it and putting it on a bench for mock up. That way I can still drive my peg leg car in the meantime.

I also got an entire 9 bolt Borg Warner posi rear end that was in a head on wreck with a telephone pole. The torque arm and the driveshaft maybe, pushed back and bent the axle tubes. I got it cheap and it had good disc brakes on it. I also got the master cylinder and proportioning valve from the same car.
The rear end also had some miles on it, so I just cut the axle tube ends off of it to possibly put in a lathe and cut the end mounts off the tubes where the backing plates would go on a drum rear. But there are also removable plates that the are the actual caliper mounts, so I don't yet know what I really need or how that's all going to work out. And I'm not sure about the side to side alignments and clearances yet either. Also, if I need to remove the drum backing plates off of the donor rearend while keeping things straight and round without hacking it up, it will be a challenge. It should be fun seeing if I can figure it out or not!

Also, I'm considering keeping the 2.73 gears in the donor rearend. My car has 3.08's in it now. For a stock dog engine, it's actually fairly peppy from a standstill. Note I said "peppy", not "fast", LoL! But I'm not getting any better than about 17.5 mpg out of the car.
So I'm thinking that if I go to the 2.73's, and either breathe a little on the 305 or put a 350 in it, I'll gain some of the lost acceleration off the line and have a little taller gear on the top end to try to get just a little better mileage out of it.

I know it would be a lot more fun to drive with something like 3.73's in it, but I want to drive the car everyday if I want to and I've got to balance performance with mileage. I wouldn't even think about this if the 700R4 didn't have the lower 1st gear to help compensate for the taller gear.

Honestly, how do you guys do it running 3.73's or so in a daily driver? At least the gas prices are down for now!

I love good high performance, but as you can see, I'm willing to give up brutal acceleration for some economy.

Thoughts?

 

Since the axle flange is a different shape, no. The drum flange is squarish with a triangle on top of the square. You can cut the triangle off, which is the shape of the disc flange and drill a couple of mounting holes, but you can't add the triangle to the square of the disc flange--at least not without it being a major PITA. Far easier, IMO, to swap the disc rear in, attach new e-brake cables, the proportioning valve, and drive into the sunset. It's a better deal all the way around.

JamesC

 

I guess I'll just be converting it over to disc then. So the only things that I need to change are the proportioning valve and the emergency brake cable? I heard that the master cylinder and the booster were different on rear disc cars.

 

Yes. Master and booster are fine. More info below:

https://www.thirdgen.org/forums/susp...fferences.html

JamesC

 

Thanks for the help.

 

drum brakes are cheaper to maintain

 

Why can't you put drum brakes on a disc brake rear end? Not all drum brake rear ends had the 3 bolt flange. The later ones had the 4 bolt flange. I've used late S10 drum brakes on several rear ends that had the 4 bolt flange. I'm pretty sure that around '90 the Camaro got the 4 bolt flange and drum brakes too.

 

OK, you got me there, but that's the ONLY advantage. IMO, the better performance of discs outweighs the difference in the maintenance costs

 

I'm willing to learn. The following is a drum to disc conversion from a 91 Firebird. Note the flange:

https://www.thirdgen.org/ls1reardisc

JamesC

 

i totally agree with you on the performance of rear disc brakes compared to rear drum but either way you put it, the braking ratio is still 70/30 and funding a better front disc set up with slotted rotors and such leaves more room to better fund the rear suspension rather than brakes. just my .02 (a good debate either way)

 

The ratio may still be 70/30, but I still think discs are the way to go, I swapped LS1 rear disks in with a 1LE prop valve and LS1 MC last summer. I did the rear brakes 1st just because I got the 4th gen rear and LS1 brakes cheap. The rear brakes were kinda an added bonus to doing the rear swap. I am collecting parts now for the front brakes. I already did the whole suspension, front and rear, with UMI stuff. So I guess it just depends on how far down the list you are. But I wouldn't waste a dime on drum brakes, use that money to help swap in discs.