I have a 79 auto. I would like to change my rear differential oil and add the posi. additive; but I can't figure out how to drain the old oil. Can anyone help?

You have to locate the fill hole, which has a recessed nut on it(right rear of carrier on mine). There is no drain hole, this is where you have to suck it out from. It is easier if you remove the spare tire and tub to gain access to the fill plug. I went to Autozone and bought a manual hand pump, which looks like a grease gun with a tube on the end. I used this to suck out all of the old oil and then put the fresh stuff in. When you put the new stuff in, add 1 quart of gear oil and then all of your posi additive. top off with gear oil. That way you are sure that you have enough posi additive in there.
Hope this helps

Changed my differential fluid last year precisely how 82dukman explained. Remove the spare and carrier, it makes it so much easier. 1 bottle of GM posi-additive is all you will need.

Thanks fellas, this will help a lot.

let us know how you make out!!

Will do dukeman, it's a little cool here right now and since my vette is parked under a carport, I have to do all my work outside. So it may be a while before I do it, but I will keep you posted.

Hello Jim !
Did it last summer took my vette to my mecanic and put it on the jack took about 20 minutes i ad my botle of additive cost $ 20.00 for the job.
if you dont want to do it and take off all the parts to get to the oil plug.
Normand


|UPDATED|12/11/2002 10:52:24 PM|/UPDATED|

Jim, Corvette Fever Magazine issue (February 2003) page52 has an article on installing a drain plug to the bottom of the differential on 63' to 79' Vettes. It looks pretty easy to do and the kit comes with the drill bits and tap to do the job. You can get info on the kit from Corvette Clinic at WWW.corvetteclinicinc.com. Hope this helps. Mike

If you decide to install a drain plug in the diff, realize that if you ever decide to have your car NCRS judged you will receive a point deduction due to a configuration change from original. ~Juliet

changed my rear fluid and used the pump you suggested. It made it easier. thanks for the tip. the posi additive was avaiable from GM for about $5.50. also i used a 5/8- 8 point socket to take the plug out. (fits the square plug). another project out of the way.