Question about rear end change and trans leak

[bwilder10h]

I just bought an 85 shortbed that an older guy owned and plan to do some rust repair/paint/wheels and eventually a new engine and have the 700R4 rebuilt.

The first week I had it, I put a new oil pan gasket on it and also the trans pan was dripping and the rear seal on the trans was leaking. Well, that bad news is it was dry on my garage floor for a couple days and now trans fluid is dripping out of the hole in the converter cover. Is it safe to assume the only thing I didn't replace is a front seal that requires the trans to be removed from the back of the engine block?


I was also noticing on the factory tag in the glovebox that shows options on the truck from GM that the rear end is a 2.73...
I didn't know they put such a weak gear in these trucks. Anyway, I want to at least take it to a 3.55 or 3.73 and would like to get posi. Right now it's open (dont ask me how I know...) and a posi would be nice for after I drop a new engine in it and replace the weak 305 which doesn't run bad, just not enough. The trans is all over the place now shifting in and out of overdrive and it's ridiculous. Also wouldn't mind knowing what 12 bolt rear ends fit it... as in what vehicles to look at in the junkyard to find a complete rear end, rather than having the gear changed and still being a single tire smoker...

Thanks for all the help. I attached a pic of what the truck looks like now. Not bad for 23 years old. Interior is perfect....

[Captkaos]

Can you tell for sure where the leak is coming from? Did you replace the trans pan? If it is leaking from the pump, you have to take it out.

Going to a 12bolt isn't going to gain you anything, save the trouble and stay with the 10 and just upgrade the internals. 2.73's were the norm in all the 81-87's. Personally if it is a driver, I would get 3.42's, but 3.73's won't be too bad with the OD.

If you really want a 12bolt, anything from 73-81 2WD will bolt in on the springs, but will require a different u-joint to work.

[bwilder10h]

Well, I originally thought the leak was coming from the front of the trans pan so I replaced the gasket. When I had the convertor cover off, the inside of the bell housing was wet but the convertor was dry.... which didn't make much sense to me.

Thanks for the info on the rear ends. So now for my stupid question of the day... if I stay with the 10 bolt, can I make it a posi unit?

3.42 might be the way to go. Most of my driving will be in town but I would like to get it out on the highway and stretch it's legs once in a while and keeping the RPM's down would be nice.

What kind of money am I looking at to get a gear change?

[Albatrossian]

There are a few manufacturers that make posi units for 10-bolts.. Powertrax and Auburn are among the best. I had an Auburn posi in a Monte Carlo SS with a 454 and didn't break it.

[bwilder10h]

Thanks for the reply. Is it difficult to change the rear end gear? Does it need to be shimmed?

Any other thoughts on my trans leak?

[Albatrossian]

Originally Posted by bwilder10h

Thanks for the reply. Is it difficult to change the rear end gear? Does it need to be shimmed?

Any other thoughts on my trans leak?

Sometimes they need to be shimmed, sometimes not. You'll know when you have it built. It's not difficult to change them, but they need to be set up right for proper mesh or you'll be replacing them sooner rather than later.
On the leak, it does sound like the front pump seal.. maybe whats happening is the the spinning of the converter is throwing the fluid off.

[xtreme80]

Shop around and see what you can find in a 12 bolt... they are pretty common.

I got the 12 bolt for my 80 shortie for 30 bucks. Open with a 3.73, and that was coming from an open 2.73. It was in perfect condition, just been rebuilt, and brand new brakes on it. 30 bucks and I gave him my old rear so he could roll the parts truck still.

The deals are out there, especially if you're not in a hurry. Yeah I didn't get posi... but 30 bucks for a nearly new rear out of a wrecked truck was awesome... still runs great.

And a 3.73 with the 700r4 isn't gonna be too bad with gas mileage... mine sucks with the 3 speed though.

[MylilBowTie]

With scrap being up around here parts are getting fewer and farther between around here.

What part of Cincinnati?

[bwilder10h]

Liberty Township (Hamilton)

[Captkaos]

I personally would go with an Eaton Limited slip over the others listed.
The powertrax can be used with a open diff though, so if you find an open diff with the gears you like you can remove the spider gears and replace them with the powertrax parts and you are done. Auburn' by design tends to eat their housing when the get old and go out.

If you are going to keep the 10bolt, you can buy a limited slip, and gears with all the part for a complete overhaul for about $1000. $130 rebuild kit, r&P $350 posi $450, $250-350 setup out of the truck.

[bwilder10h]

Sounds like I might be better off to find a different rear end.