I have the differential out of the car - 1977 with 87k original miles. nothing wrong with it that I now of, but it is almost 45 years old now and the engine is producing 370 +/- hp now - Van Steel wants $750 to rebuild it. Do they fail due to age? or am i wasting the money to rebuild it if nothing is wrong??

thanks,

Michael

 

More than likely the short answer is yes. The stub axles on C3 corvettes are known issues that they wear and the play in and out is an issue. The posi clutches are more than likely worn out also and your posit reaction is probably not doing what it is supposed to.
I’m sure Gary GTR1999 will chime in on this as he is the resident expert on diffs.

 

Since you have the unit out, I would recommend pulling the cover and taking a careful look. As gordonm points out, the stub axles are highly likely to have worn. Measure the gaps between the axle tip and the cross-shaft. In the worst cases, the axles have worn to the point where the retention snap ring has fallen off.

Check the posi case carefully for cracks, particularly at the corners of the “window” and the cross shaft holes.

Look at the spider gears for wear on the tooth contact surfaces.

A “rebuild” from the vast majority of the vendors will get you new bearings and seals but nothing else, save for possibly a set of clutch plates which they will likely replace with fiber coated plates (not good).

A proper rebuild will include a full detailing (deburring/radiusing/blending/polishing) of the posi case, lowering the face of the cross shaft retention bolt, steel clutch plates and, most importantly, “tuning” the posi (go to YouTube and search for Tom’s Eaton Posi Tune).

It will also include Timken bearings and ARP ring gear bolts.

A bit of good news for yours - you have the 3rd gen posi case and 17 tooth spiders so you have a great foundation to work from.

As gordonm points out, Gary Ramadei (gtr1999) is the expert on these. Search this forum for the many posts by him on this to do this correctly. I built mine to handle my clone L88 and Gary was instrumental in my learning process.

 

How much do you drive the car?
You said it's working fine. If it ain't broke, don't fix it.
Repairs on these old cars are always iffy, what with quality of "new" parts, mechanic's skill level, etc.
Change the gear lube and add the posi additive as recommended.
Drive and enjoy, don't go looking for fixes to problems you don't have.

 

I just took my '72s posi apart yesterday and while it was working ok, I found the axle tips were worn and grinding into the cross pin. I was going to have it rebuilt anyway while the whole car is apart, but these findings confirmed (to me) that I was on the right track with rebuilding it now rather than pulling it out again later. As some of the others have said, look up Gary/ GTR1999 posts on the units.

 

If youre going the rebuild route, I suggest changing the gearing at the same time depending what you have in there???? Prob a 3.08, I'd recommend the 3.70's

 

I'm in the middle of doing the same thing on my '79. How do I locate Gary/GRT1999? I need the set up specs for end plays, runouts, etc.

Thanks,
Al

 

He's on here often so he may see this thread. Or send him a PM..

 

thanks for everyones input!! answers to some of the questions below...

i have the 3.36 rear end and 4 speed transmission. with the 4 speeds, the cars rpms is higher than I like already at highway speeds with the 3.36 so not inclined to change the gear ratio unless I upgrade to a 5 or 6 speed transmission, which I don't intend to do at the moment.

Car is weekend driver only, but I did increase the hp for a reason because there are times I like to drive it hard, but mostly just cruise around with it.

Is there a place that rebuilds them "properly" per 69L88's comments?

There is some play on the two output shafts, about twice as much on the right side than the left. There is no play on the input shaft. Based on some of the comments, I would feel better if someone who knows what they are doing went through it.

thanks again,

Michael

 

Its GTR1999

 

Here is a link to my diff rebuild thread:
https://www.corvetteforum.com/forums...iff-build.html

I am no expert, this was my first diff, but I learned a ton from GTR1999 (especially) and a few others along the way.
Notes:

• Your 77 yokes are probably the soft ones which wear badly. They should move less than .120" in and out of the case while assembled. More than that inspect the case for damage where the yoke could hit it.
• After dissasembly they should measure .189" from the c-clip flange to to top edge.
• If not plan on buying new ones.
• Inspect the posi case window corners and cross-pin hole carefully for cracks. I found four cracked used posi housings before I found a good used one.
• Then Gary's threads have a list of things you can do to it to beef it up, if you want. I think I have some links to his threads in mine.
• ARP ring gear bolts are mandantory, IIRC some of the ring gear bolts in the late 70s diffs would back out on their own.
• 370HP is enough to break it if you drive it very hard or use slicks. It would probably be OK on the street without big clutch drops.
• Use the search tool, search for Garys name "GTR1999" and "diff". You will find lots of info there.

 

Happy New Year guys, hope it's better than the last two.

Some points to Leighs notes

• 70-79 original axles are typically worn once they reach 45-50k miles. Currently, rebuilt axles on sketchy on quality and if they're even in stock.
• The measurement from the face to clip varies, according to wear or if they are rebuilt. Generally, .185-.200 will work well in a Tuned posi and may even need to be dressed in a surface grinder.
• ARP RG gears are all I use and you need to use Loctite on them
• Breaking parts depends on a few things. First NOTHING is bullet proof. You can turn a nice stock posi into instant junk with 300 hp and hard abuse as easy as 650+hp. We have bent 12 bolt axles at the track. I have seen 12 bolt, 9" and Dana diff's break. The old saying is still true= you will pay if you want to play.
• IRS is not the best for a drag race setup, it can be built to handle a lot but if you are going to have 2x the power the car was designed for, abuse it at the track you need to look at various options, all have their place. A few places have come out after Toms business since his death. I think they're low lives myself- others will say it's business, but those who plan on power need to do a lot of homework or you will be throwing away a lot of money
• If you think builds are the same and only the price is different, you're wrong and will end up paying more for less. Again, education is your best option. Trying to compare a wrench build, done in 8 hours, to a machine-fit build is nowhere close to each other. Leigh knows about this first hand.

Good luck, those doing them at home check your parts and don't be surprised if they don't fit. Case in point I just finished correcting a common rebuilt diff- it was sold with a cracked posi case, loose lash, and stripped out pad holes. It took me 40 hours to fix this mess. If you miss items like this, you will be doing it again when the diff implodes and leaves you stranded.

 

Wow! thanks for the compliments Gary. I can think of no higher praise for my first ever diff build.
And I definately could not done it without your frequent advice, and even your hands-on help at the end.
I now know why my 500+HP BBC would have quickly shreaded my stock diff or even a normal "bolt-together" rebuilt diff.
For instance I could not believe how large the "lash" was on the spiders in a stock diff.
But yeah I probably have 100+ hours into my machine-fit home rebuild.
I basically "spent" the same amount of money as buying a stock rebuild diff, and hopefully mine is far stronger.
BTW:
I still have the diff blue-printing tools available:
https://www.corvetteforum.com/forums...r-program.html