Shown are two posi I am building on the bench now. This is not a solicitation, these are not for sale. They are going into diff's being built to the application and clearly the applications are different.

Both are 10 bolt, which all 63-79 corvette posi's and open diff's were.

Both are running 10/17 spiders, one NOS Eaton 12 bolt -30 spline net forged and the other original Eaton 10/17 x17 cut spiders.

The 30-spline set up requires several machining steps to complete. The 17 spline is a stock set up, still I do some machine mods to them.

Both are Tuned, meaning- no lash, no binding, no axle endplay, no chattering, they are balanced between the sides, and yes, they operate in posi action.

The 30 spline posi and parts will be cryo treated, the 17 that is not required by the application.

They will perform better than a production built posi. Both use solid steels only- no fiber or snowflakes. All three clutch types are sold today, even though some are weaker than others.

They take hours to correctly build compared to 15 minutes building a stock spring loaded setup.

I have literally built 100's of these, most likely more than anyone else's other than Tom's, but since they stopped building posi's 15 years ago maybe I am close now. It doesn't matter.

This is not a "how to" thread, nor will I go into great detail on it. For those that care to debate, I have no interest so please don't bother to respond since I will just ignore you.

For those that want to try to do this, go for it but know this. If you don't correctly build the posi, then it will not be balanced and it will either not function as a posi or you will bind the spiders and break them.

 

Gary, as you know, I support what you do 100%. Your work, your knowledge and your integrity is rare in todays world, especially in Corvette repair world. I am happy I have had to pleasure and success to use your work on the trailing arms and the differential. If I get this engine built, I will be running the differential you built me this summer.......with a TUNED posi. Changed my forum Username recently....but you know who I am.

Jeff M.

 

Yes I do. Thank you

 

What are the "spiders"?
I think that lets you know I won't be trying this myself ha ha

I know nothing about rear diffs but I sure like looking at what you build
Thanks for the post Gary

 

No problem. The spiders are the small gears inside the posi case. The small ones are pinion gears, the larger ones side gears. The side gears are splined to match the axles, since that is where they fit.

 

Where do you get the shims that fit behind the clutch packs, I think I need to do some work on mine.

 

I stock all the parts I use.
When tuning a posi, the correct way, many times I have to grind shims to size. The 30-spline ended up, pre-CYRO, at 032" and 0335" The shims come in 005 increments. Is there is difference between using shims within 001-002" = yes.

 

Gary - always enjoy and learn something from reading your threads. Thanks again for rebuilding my steering box.

Had to look up cryo treating. Do you do this yourself? Once cold, how is it brought back (controlled) to room temperature?

Appreaciate the sharing of your machining wisdowm with us.

Fran

 

Hi Fran,
I did your box 6 years ago already, time flew by.

Cryo treating has been around a long time and it is not limited to the automotive market- same as REM polishing wasn't a process started for automotive.

I do not do the treatment. As with heat treating, I use those pros who do it every day. It is a 3-day process- 24 slow freeze to -300*F, 24-hour soak at that temp, 24-hour slow thaw. Sometimes condensation will flash the metal so I may have to repolish the parts when I get them back, other times no change in appearance.

 

One noticeable thing I noticed is that my posi housing rang like a bell after the cryo treating. Just like good hard tool steel does, but even more bell like, That would have been due to the round shape.

It did NOT do that prior. So the metalurgy changes, total hardness, or consistency of hardness thruout the posi caused that. It was very clear and sharp, one musical note, like a medium "C" or "F", meaning one frequency and one natural harmonic vibration level. The original must have had multiple harmonics, due to inconsistencies in the microscopic grain structure, or metalurgy, so they cancelled each other out and you could not hear the "ring". That is exactly the type of changes that cryo is famous for, removing inconsistences, or weak points, in the metal.

I am a musician and I found it very interesting. Trumpet bells have a similar phenomenom. The metalurgy affects the quality of the musical notes. Consistency is the key. And that is what separates a beginners trumpet from a more expensive professional one, the metal itself. And yes they do sound different, "clearer".

And Thank You to Gary for all your assistance to me and this forum!
I could not have done it without you.
Your insights have kept many of these beautiful cars on the road!

 

Keep up the honest working ethics you have!

 

I don't have the right to copy articles out of magazines but if anyone has the June 1979 issue of Popular Hot Rodding there is a great story on Tom Watt and posi tuning. It's one of the first ones I read that gave a good explanation. Tom bought the business from Pepe and opened Tom's in 1975. 75 was the first year I learned about posi tuning. I still have that mag and the one the one from 1975 that started me on the road.

There are some on ebay for sale now in fact.
Popular Hot Rodding, June 1979 | eBay

 

Thanks Gary for the explanation. Aways learning something new here!

Fran

 

I see the magazine on ebay sold, someone here buy it?

 

Yes. thanks for the tip Gary!
Should be a good read.

 

Oh, good deal, please read it and post your thoughts of it.I have to pull it out and go over it again, it's been years since I have read it. I used to get that mag and several others in yearly subscriptions. I recall the day that issue came in and the story about Tom and tuning was in it. I had my 67 coupe back then and was only 19/20 but the diff was good so I never pulled it from that car. I still have the mag but would rather have the 67 back- NOM and all!

 

Tuned posi in action, this is from years ago. My son's car. Both wheels locked up just a like a spring loaded posi would have. That car ran a spring loaded posi when he first got it, until he blew the differential. I built him a 12 bolt- with tuned posi, first 1350 axles but those are not for racing, then 1480's. Of course today, the only place that has the original 12 bolt 373 gears made for Tom is me. The 373 and 411 are gone. Tom's had some 456's the last I checked.

There is a new vette 12 bolt build video on YT, I just saw it and saw some things questionable in it. Anyone here part of that video?

The 2nd video is of the 2nd 75 vette. A pizza kid ran a stop sign and took out the white 75.

Many think I also drive my cars like this, but I don't. For me all I like is a summer day or Fall day, away from cars and people, back roads, nothing to prove to anyone.

The point, the spiders never broke, the clutches never worn out, but after 9 years of abuse the 12-bolt pinion broke a tooth. He still has the vette but is running in the 9's with a Mopar these days.

My own 72 with tuned posi only needs to have regular oil changes about 8-10k miles. Driving with a tuned posi doesn't feel any different, in fact it's smoother than a spring setup.

The new fiber clutches don't chatter, but they smudge away at every turn. Also, with the fibers you CAN NOT tune them.

Most of the people who told Pepe, Tom, and me that a posi won't work without a spring pack usually never built one and we always bounce back to the "engineers didn't design it that way" answer. That might be because they don't know how to build one or don't want to put in the time to do it correctly. Some of the conmen out there who took some guys here and elsewhere said they tuned the posi but when the customer posted pictures it was a spring pack setup!! One of those times Tom was still alive, so I shared it with him- he laughed so hard I thought he was going to get sick. So, you can't do anything unless a production engineer from 60 years ago approved it? I will have to remind all the guys with LS engines in their cars that they won't work since one of Zora's guys didn't draw it up. Same for using 31 spline outer axles, correcting runout in rotors, machining steering box parts, etc, etc,etc.

One thing I noticed over the years the debaters on many forums usually don't contribute any of their own material. Interesting that some may have all kinds of engineering and technical advice to offer for someone else, but they never share what they are doing? Why is that? I say this without any one person in mind since I have seen it for over 25 years online now. The one common theme, they like to latch on to something and beat it to dead as long as they have the last word.

 

Thanks again Gary for sharing your knowledge,,, and photos of your current builds. I am so grateful to have one of your rebuild & tuned differentials in my '75.
,,, I just grin ear to ear when I talk to so-called rear end guys. I never argue, just let them talk. Sadly, most don't want to hear the truth.
.

 

So, the real question is how do we get Gary (and Lars) inducted into the National Corvette Museum?

 

Great question for these guys recognition,, but I personaly believe they are better than that! ,,,