Posi Rear Differential Gear Oil....

[Ala75Vette]

While I've got most of the rear end disassembled getting the trailing arms off, I want to check the rear diff oil level, and add some if I need to. I'm reasonably certain I need to, since the rear seal in the diff looks basically "shot" and calling it "juicy" back there is an understatement. I recall from reading posts in the past that you want to make sure to use "posi-traction" diff oil, versus just regular-old gear oil. The "salesman" at Advance Auto convinced me to buy some "Trans-X, Posi-Trac, limited slip gear oil additive" to mix with regular gear oil, to wit I bought some "Valvoline, Durablend, Synthetic blend, 80W-90" gear oil, with a GL-5 rating (a bit to do from Mr. Advanced Auto). Opinions on whether this sounds Ok? I'd love to just have the rear differential rebuilt, but I'm hoping I can get a little more life out of it before forking over those bucks....

 

[Tom B Hargrove]

Go to your chevy dealer and buy two bottles of posi-traction additive. Drain your diff. (you have to pump it out), and put the additive in . Then top it off with gear oil, 80 to 90 wt. If your leak is too bad, you might want to replace the gasket while you have it down. That way you could empty it much easier than pumping it out. When you fill it back up , just fill to the oil begins to come out the fill hole. Tom

 

[GTR1999]

I've used the Valvoline oil and it will cause the posi clutches to knock if you don't use the GM additive. If you have the rear suspension apart now and you have a seal problem I suggest correcting it now if you can.
You have a 75, has the differential ever been worked on that youknow of ? Is the differential still mounted in the car?

You can replace the front pinion seal in the car but you run the chance of not getting the proper bearing preload once you remove the pinion nut,even if you try to get it back in the same position.

Add any wear on the side yokes and you could have a problem developing there. With the 1/2 shafts out of the way check the end play in the yokes. It is best to use a 1" dial indicator for this but you can use a feeler gauge. If you find more then .0625" )1/16) end lay I would drop the differential and open it up for a look.

 

[Ala75Vette]

Hmmm. So I guess it ain't natural for those diff side yokes to have any play in them, huh? Pulling on the passenger-side half shaft, I can get some noticeable play, I'm guessing about 1/4 inch, without measuring. I'd go ahead and replace the rear seal, but am I wasting my time? Ugghhh. I feel a bit uneasy about rebuilding the rear end myself, especially after having read the "how to" at corvettefaq. How pricey is it to get a 75 3.08 rebuilt?

 

[GTR1999]

1/4 is way too much,the snap ring that holds the yoke in place is .187" from the end of the yoke so it must now be in the differential somewhere.

A standard rebuild where you drop off or ship the differential should run about $500. That SHOULD include bearings seals, posi clutches, set-up but not new yokes,posi gears,or ring & pinion. It cost me about $600 to rebuild a differential when I do them. This includes the above parts,plus new ring & pinion,HD cover, and yokes. It does not include any labor,which could run a lot more if I charged for the tiem I put into them.

I think it would be a waste at this point to ony change out the seals. Drop it out and drain the oil. Check it with a magnet for metal. Mount it to an engine stand if you have one and you can inspect it. I can wlak you through it if you want.

 

[Ala75Vette]

Thanks for the advice. As painful as it may be, looks like I'll be dropping the diff, too, while I'm under there. Just don't tell my wife I've got an extra ~$500 coming up soon.... Thanks again, I'll keep you posted as I get estimates & pull things apart.

Frank