Hi everyone. This is my first post. I have a 67 RS/SS 350 with a 4.10 12bolt posi. I am in the process of changing the differential fluid and I need some suggestions on the type and weight of fluid to use and the correct amount. Right now I have 2 qts of Royal Purple 80w90, but I haven't used any yet. I wanted to be sure before I put it in. Also, I am not sure what type of posi unit the rear has in it. The rear was already assembled when I bought the car. I do know the guy paid quite a bit of money for the rear. Around 12 or 13 hundred. Is there a way to distinguish between one posi unit and another? Any help would be greatly appreciated. Thanks guys.

If you have the cover off then you can look at it and tell which one it is. if it has 4 springs in the middle then it is probably an Eaton. If it has 5 springs then it is an Auburn. Use the 2 quarts that you have with a bottle of GM or Ford limited slip additive. If it is an Auburn then use 2 bottles of Ford additive.

Look at these pictures and see which one looks like yours.
http://community.webshots.com/album/55410292AnMJrE

Thanks BGH. It won't be overkill if I add the limited slip additive when the bottle says works for limited slip and posi will it? In your opinion, which posi is better, Eaton or Auburn?

You will probably need the additive anyway. The Eaton is far better than the Auburn. I won't install an Auburn unless I have to.

The 80W90 is what I am using...

thanks guys. i do have one further question. when i said posi, i am assuming it has one because when i punch it both tires smoke and they both leave patches of rubber on the ground. also, when i go around a sharp corner with the steering wheel in the full lock position, the tires chirp. thanks again guys for your help.

Probably a posi, but could be a mini spool. When you remove the cover take a look at it and see what it is.

What is a mini spool and are they any good?

Does anyone know the part # for the GM additive? I am going to the dealer today and my dealer is pretty rude. If I walk in and ask for a limited slip additive they are going to be like WHAT???

The part number is GM 1052358. It's about 7 bucks for the bottle.

http://community.webshots.com/album/55410292AnMJrE
Look through these pictures and see if any of them look like what you have. The mini spool isn't made for street use. It is for dirt track use, but many people run them on the street. They should not be used on the street.

Thanks Lucky68. I'll be on my way today. Hey BGH, if I do have a spool can any damage occur? My dads friend is the one I bought the car from and he built it for the drag strip so it's very possible that it has a spool. Maybe not a mini, but I was watching Horsepower TV and they referred to a "Spool" and said it was for drag racing. Not too good for street, but if I wanted to put as much of a direct line of power to the ground then the spool would do that. What do you think?

I know that many people have run spools on the street with no problems, but I have seen broken axles, broken wheel studs and other problems from running a spool on the street. There needs to be some kind of differential in the rear end that allows one wheel to turn faster than the other when going around a corner. When you have a spool it puts a lot of stress on axles, wheels and other parts when you turn a corner.

Thanks BGH. I opened up the rear end this weekend and I found one of the carrier shims was sliding out. I'm not sure why this happened, but I tapped it back into place. It sisn't take much effort at all to put it back in. I wanted to pull the axles, but I didn't know the torque spec. or exactly how to do it. Well, I saw that my rear definately has a posi unit. It has 4 springs. Two on one side that are close together and two on the other side that are spaced about an inch or so apart. Maybe an Eaton? I'm not sure. I'll admit, I don't know anything about rear ends. I replaced the fluid with Royal Purple 75W-90 and added a 4oz. bottle of GM part #1052358 limited slip additive. I also installed a new cover. It is a TA brand. Let me tell you, this thing is beefy. It appears to be made of cast aluminum and has load bolts and a drain bolt. I had to get rid of the nasty, rusty chrome cover that was on it. Well anyway, since the fluid change I haven't heard any noises or anything so I hope all is well. Thanks for all the input.

Hey BGH, do you what the reason could be for the carrier shim to be backing out? It didn't fall all the way out, but it did come out enough. Thanks.

The shim slid out of place due to power being applied to the diff. When power is applied, the diff has a tendency to go towards the drvr side due to tooth design and engagement, thus loosening up the pass side shims.

No problem done, and it can't come out due to the axle going through the center of it. Only problem is the inside edge of the shim is chewed up, the shim may have been saved and no further damage done.

cool. thanks.