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I’ve got some questions regarding the differential of my 77 Vette.
A) I will remove the cover. What are the most important parts to check inside (noob here, never did this…).
B) What do I use to clean the inside (safe)?
C) What gasket do I best use when sealing it? (Felpro?)
D) What fluid do I have to use? (Belgium here… Does it have to be ACDelco stuff?)
Check these....there are two of them....sometimes they become bent/out-of-round and the fall out of the splined shaft grooves .........
Snap ring is still in place where it should be.....
The other Snap ring has been damaged and has fallen out of it's groove......
I found the damage Snap ring before it could do serious damage to the internal gears.......
The grooves in the splined shafts were not machined deep enough by GM (probably a dull cutting tool) according to the Machinist's Handbook......I did not have access to a metal lathe so I took my time using a diamond dremel wheel to deepen the grooves in both shafts. You can see how the Snap ring barely fits into its groove.......
A Machine Shop could have done the modification in about 15 minutes.
Last edited by doorgunner; Oct 3, 2021 at 06:06 PM.
A lot depends on if the diff was ever worked on and if it was done correctly. If it has never been apart then 77 diff's have some areas to check.
1- Ring gear bolts- check the type used and torque them. You can also replace with ARP but you're overseas so I don't know how efficient that will be.
2- Axle faces- 77 will have soft face axles and might be worn done. As they wear the axle endplay increases and the IRS handling decreases. Axle end play on a stock diff from that era can be all over the place. If you find 040" or more I would look at replacing the axles and checking the posi. If you have never worked on a diff before then the posi work may not be where you want to start to learn
3- check the posi case for cracks, look at the radius of the large window, the cross shaft holes, and back window.
4- Clutches- 77 will have the weaker snowflake type
The best book is in my head, sorry but after doing these for many years there are too many things not in a book. However the GM service and overhaul manuals are the best I think. You can find them online and ebay.
There are many opinions on oil, I use Lucas 85-140 gear oil in diffs over 30* F, otherwise 90wt. 2 bottles of GM additive.
The best book is in my head,
There are many opinions on oil, I use Lucas 85-140 gear oil in diffs over 30* F, otherwise 90wt. 2 bottles of GM additive.
Multi
#5 GTR1999's Avatar GTR1999 , 01-10-2009 08:45 AM
Tech Contributor
" ' "All the diffs I rebuild use 1 bottle of GM posi additive & Lucas 85-140 gear oil- non synthetic.
90 wt gear oil will work but I like the Lucas better. Regardless, no oil will save a stock diff with wear on it. If you have a 74-79 check the yoke endplay and the RG bolts, along with the case for cracks. The 65-68 were real bad for cracking, the 69-73 were some of the best but had lousy clutches." ' "
I feel compelled to ask, GTR1999, Why you now recommend TWO bottles of additive instead of ONE as you mentioned in a older post... I just recently did mine with one,,
The GM part number and packaging changed about 10 years ago. I can't say for sure on the exact date or if the chemistry of the product changed, I can say I had posi chatter in a tuned posi for the first time when one bottle of the new part number was used. I changed to 2 bottles ever since without a problem.
Thanks, good to know. I was not haveing any issues, I had it appart to change a broken cover. I'll pay close attention to any sound , and adjust the additive amount as needed.
Good info!
A lot depends on if the diff was ever worked on and if it was done correctly. If it has never been apart then 77 diff's have some areas to check.
1- Ring gear bolts- check the type used and torque them. You can also replace with ARP but you're overseas so I don't know how efficient that will be.
The first one is wrong size, the 2nd one are ring gear bolts and the correct ones. The cover bolts are 7/16-14 x 1.250", pretty common from most suppliers. You can use grade 8 hex or socket heads with narrow lock washers. If you change the ring gear bolts the flange holes have to be countersunk and Loctite #272 used.
The first one is wrong size, the 2nd one are ring gear bolts and the correct ones. The cover bolts are 7/16-14 x 1.250", pretty common from most suppliers. You can use grade 8 hex or socket heads with narrow lock washers. If you change the ring gear bolts the flange holes have to be countersunk and Loctite #272 used.
Thank you! I have one last question:
What gasket do you use putting the cover back on? I don't want any leaks.
The best book is in my head, sorry but after doing these for many years there are too many things not in a book. However the GM service and overhaul manuals are the best I think. You can find them online and ebay.
There are many opinions on oil, I use Lucas 85-140 gear oil in diffs over 30* F, otherwise 90wt. 2 bottles of GM additive.
Another option for lube is the original GM bottles and GM posi modifier from Amazon. Usually cheaper than others if you play the shpg game.
Does Amazon have a warehouse facility in Europe?
The cover gasket, I found at Corvette Central here in the States for around $15. By they time they ship it to you, it would be $5,000.
You can try EBay for a gasket. Amazon ships through EBay using their packaging.
Last edited by HeadsU.P.; Oct 8, 2021 at 03:29 PM.
Differential questions
