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Old 06-02-2007, 04:37 PM   #1
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Posi Tuning

OK another thread in my on going series of C2 & C3 Tech. This job is for my buddy Paul. He may post some pictures if he sees this as to why he needed a new Posi.

First a little background for you guys who have never looked into a vette rear end

I think everyone knows the '63 were the first with IRS. The Diff's were cast iron and posi was an option. Most had it but there are still some open diff's out there. The first posi design was total junk and didn't last long. I see rebuild kits for these on ebay from time to time but if you have a 63-64 with that crappy posi, toss it and get an Eaton. Eaton's were first used in '65 and went thru to '79 when GM switched over to the 80-82 aluminum diff. These are also junk in my opinion but this is not about them.

This thread is about making a Eaton posi in a 63-79 vette better. The complete differential rebuild will be in another thread at a later date. We are going to assume you have the posi out and it is either going to be rebuild or replaced. If it is cracked then it is junk and you will need to replace it. Forget about that excellant welder you know, toss it out.

Here is what a "stock" eaton posi looks like in your vette.



Now you can buy a loaded Eaton 3 series that will accept a 273,308,336,355,370.373.390.411, or 433 gear. I don't like the loaded Eatons because they use fiber clutches, HD springs, powder metal gears. Ok for a low performance car but not a vette. Keep in mind this is my opinion as well.
I have used a couple where the owner was cost conscience. Here is one.



I'm going to show you how I rebuild the Eaton cases into a better unit. No doubt a shop or vendor will not build them like this and may even tell you this way is no good. I really don't care what they say,let them cry to someone else you will know more then they realize.

This procedure is not mine, I first read of it back in 1975 when a guy in CA named Pepe Estrada was building vette drag diff's, he later sold the business to a guy named Tom as many of you have heard of. If you have access to old magazines this was reported on in the June 1979 issues of PHR. I still have it I would post it but it would probably be copyright violation.

OK on with the show. I going to build a new posi with all new parts.

Here is a brand new Eaton case. It is very nice but you'll see the opening are rough cast. This is what a shop would use as is. We're going to work it over a little bid. I polish and grind them to a smooth finish. The thought here it to break all the sharp edges and smooth out the seams. I won't go as far and use that over rated term of "bullitproof" as there are a lot of bullits out there.












As you can see they were rough. I start with a burr and then move on to various stones,wheels, & emery wheels. A head porting kit is what you'll need if you don't have anything but a die grinder. Summit has them. I finish it up in the lathe.







I blend in the edges and corners into a baby smooth finish. When you have it like that it's time to move on to the spiders.

Now the guys who followed my posts for years hear me speak of 10-17 or 10-18 spider gears. These are the gears inside the posi case. The posi clutches are stacked on them. The 65-70's used 10-18 gears and they were not too good. I would normally call them junk but these are now classics so I don't want to offend anyone, if it was 1978 I'd call them junk! Also the early cases were not as good as the ones pictured here. If you have a little square window in yours it is an early one and they tend to crack so look it over good and spray the edges with brake cleaner to show up the cracks.

Here is an early case with a crack in it.



Here we have a 17 gear and an 18 gear. The 18 gear has a cast in flange and is a good way to ID them in the diff.





Here is another 18 gear from the side



I think I'm close to the picture limit again so I'm going to end this thread and start part 2. I know Jason will combine them for me.

Last edited by gtr1999; 01-02-2011 at 10:11 AM.
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Old 06-02-2007, 05:05 PM   #2
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POSI TUNING Pt 2

Ok, I'm back

You should now have your Eaton posi case all nice and smooth, have good or a new set of 10-17 spiders and now we'll look at the clutches. I forgot to mention I do not use those crappy 4 springs and plates. Those are the reason so many posi's hammer around corners. If you set it up like this you will eliminate that problem.

The 65-70's used solid clutches. They were great but with the production tolerances of .000-.008" for the spider backlash those springs and plates were used to keep constant force on the clutches. You had posi yes but try an take a corner and keep your teeth in your head So the clutches were changed to the slotted type that I call "snowflakes" the thought was more additive would get in between the clutches and stop the hammering. Yeah right bubba, all this did was to weaken the clutches and they still woudl hammer only know they would break.

Here is a 74 posi I rebuild. Every clutch was broken in it.





So what to do? Well some vendors are still selling the snowflakes, or you can go with those wonderful fiber coated clutches. I had a new set and they smudged in my fingers. Do you want to use them in your car?

Here is a new fiber clutch



Here is the same clutch after I rubbed them with my finger



I use solid clutches only. If someone wants one of those fine examples above, good luck.





I coat them with GM additive and only GM additive. I also only use Lucas 85-140 gear oil in the diff. You don't need any fancy sythetic oils. A US Gear engineer once told me to stay away from the sythetics for differentials. I stack them with the deepest stamp in one direction to allow a little more room for oil/additive.



Once stacked and oiled I start with a .025 shim on each. This will vary with each posi case so if you start with a .040 it may be fine. The last 5 posi's I built ended up using shims in the .030" range. This one ended up at .040"




Now assemble the gears in the case, use new washers. Here are all the parts going into Pauls case.



I set the case on a side yoke and 3 old yoke bearings. This gives you a great setup stand to work from. Once assmebled turn the case and see how it feels. If too loose then increase the shims until it binds. then back off to the next lowest shim. they come in .005 increments. What I do is back off then grind a few shims in between the two so I can dial in to where I want them. This takes some time and you will need a surface grinder if you want to dial them in. This one was .040 per side but sometimes they are not equal in shiim thickness. Also you may find your spiders are worn and you will not be able to set them up. I once had a case that was machined off and I could not tune it this way. It was only good for the spring setup and it was tossed for a new case.

Here are some of the shims I ground to use in this unit.



Here is the finished product, ready for the ring gear and installation in the housing.











This took me all day to do so don't expect to do it in an hour. You can do it and you may not need the surface grinder. I have walked a few guys through the job. I got them the parts and instructions and they are on the road today. I have never had one of these hammer and there is less wear on the clutches. So now you can think about it if you get to the point of this on your vette.

Have fun , it does work.



Next up Differential Blueprinting. My buddy Mike is waiting for his 68's diff to put it in the car.

Last edited by gtr1999; 06-04-2007 at 10:08 AM.
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Old 06-02-2007, 06:07 PM   #3
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Gary,

Looks Great! I can't wait to put it in! Got the new 3.36's on order.

For those of you wondering why I need a new posi, here is a pic of my old one. Take a look at the long crack at the bottom of the large window. It goes all the way through to the inside of the case.

I discovered it when I was replacing my yokes. They had way too much play in them......simple yoke swap turned into a completely rebuild rear end, reworked halfshafts and drive shafts. Cleaned and painted everthing.

I am saving the trailing arm rebuild for next winter.

Paul




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Old 06-03-2007, 03:41 PM   #4
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Quote:
Originally Posted by gtr1999 View Post
I think I'm close to the picture limit again so I'm going to end this thread and start part 2. I know Jason will combine them for me.
happy to help, Gary. This is exactly what I'll be needing next winter when I tear into the rearend.
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Old 06-03-2007, 07:00 PM   #5
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Thanks Jason, I knew you would fix it up If you get into a hassle with yours let me know and I'll walk you through the job.
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Old 06-12-2007, 02:13 PM   #6
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good thread
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Old 03-09-2008, 04:43 PM   #7
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I just tuned an early posi case(65-68) and it took more time then usual. I found this casting was not allowing the pinion gears to rotate freely out of the case. The lip of the countersunk area for the cross shaft washers was too large and the gears caught up them. I had to dress the end of the gears and blend in the lip in order to setup the posi. So if you have an early case check for it.

Every job is a little different so if you're following this for the first time expect things may be different then I show here. You may have to make some decisions at home on how to proceed. If you're not sure-stop- and find out before you wreck the case.

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Old 04-27-2009, 09:55 AM   #8
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I just tuned an early posi case(65-68) and it took more time then usual. I found this casting was not allowing the pinion gears to rotate freely out of the case. The lip of the countersunk area for the cross shaft washers was too large and the gears caught up them. I had to dress the end of the gears and blend in the lip in order to setup the posi. So if you have an early case check for it.

Every job is a little different so if you're following this for the first time expect things may be different then I show here. You may have to make some decisions at home on how to proceed. If you're not sure-stop- and find out before you wreck the case.

GTR can i replace my aluminum 82 differental for a 79? will it be a strait bolt on? I to plan to rebuild my rear and thought about having u do it. Just wondering if you have a website or something where i could get some pricing
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Old 04-27-2009, 10:19 AM   #9
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You can replace the complete differential with an iron unit from 63-79. The conversion will require using your rear cover and the iron unit strut rods,1/2 shafts, and brackets. The front bracket changed over the years and some do not fit well. also the DS will probably have to be fit.
This will be costly to get the iron unit and all the parts so think it over good.
You can PM directly with questions and I can give you some options.
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Old 04-28-2009, 05:22 AM   #10
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You can replace the complete differential with an iron unit from 63-79. The conversion will require using your rear cover and the iron unit strut rods,1/2 shafts, and brackets. The front bracket changed over the years and some do not fit well. also the DS will probably have to be fit.
This will be costly to get the iron unit and all the parts so think it over good.
You can PM directly with questions and I can give you some options.
well the only reason i mentioned it was u said the aluminum units where crap. I would prefer to keep my aluminum unit and rebuild it to handle around 650hp and about the same for torc (the cue key dont work on this computer lol)

If you think u can build my 82 rear to handle that kind of power than i will just use that one.

Maybe u can answer this for me to, I saw places that make carbon fiber driveshafts and half shafts, but they dont recommend them for drag racing? arnt they suppose to be stronger than stock? I plan to use mine as a weekend warrior and would like to find myself in the 11 second range, so i need a drivetrain that can hold up to that. but not one that will eat my pockets eaither

I will send u a PM this is probably something I dont plan on doing for a while though, I also need to get some offset trailing to
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Old 04-28-2009, 05:59 AM   #11
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EC,
You will get various opinions on the 80-82 differentials. Some run them with over 400hp and they have no problem, others have broken with the stock engine under 200hp.
Yes I do not like them, wouldn't like one on a garden tractor let alone a vette. That's just my opinion, I'm sure others will disagree.
Building one for 650HP, no way on earth I'd even attempt it. I would not use a iron 10 bolt at that level, you're into 12 bolt conversion land then and the 12 bolt won't work with your rear cover so you would have to fabricate the complete rear mount of the earlier cars.
You'll have more into the rear alone then the car would be worth if you ever want to sell it.
I'd look at another year or model if you want to really push it hard.
Bottom line is it is your car and money, throw enough at it and you'll get to where you want.
Good luck, think it over good.
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Old 04-28-2009, 06:51 AM   #12
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Great thread, I eat this stuff up!

Few years back started setting up my own gears and have been wanting to take the step into limited slip rebuilding but have learned there's not much information out there. Thanks again.
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Old 04-20-2012, 08:55 PM   #13
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FYI
If you haven't seen it, Tom has posi tuning on You tube.
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Old 04-21-2012, 06:12 PM   #14
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those are good videos! thanks for the tip-off. didn't think that Tom's Differentials had embraced the information age...
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Old 04-24-2012, 06:36 PM   #15
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Well after years of doing these by hand I finally finished building my posi tuning machine. Not as fancy as my buddy Tom's but I made it from scrap and a machine I took out of the salvage bin.


Last edited by gtr1999; 04-25-2012 at 06:23 AM.
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