Corvette Forum : DigitalCorvettes.com Corvette Forums banner

1 - 20 of 29 Posts

·
Supporting Vendor
Joined
·
3,770 Posts
Discussion Starter #1 (Edited)
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:lookinup:

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 rebuilt or replaced. If it is cracked then it is junk and you will need to replace it. Forget about that excellent 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 308,336,355,370.373.390.411, 433, or 456 gear. I don't like the loaded Eatons because they use fiber clutches & HD springs, the spiders are not the same as the formed 10-17s used from 71-79. Ok for a low performance car but not a vette. Keep in mind this is my opinion I'm sure others may disagree. 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:crazy: 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 used 10-18 gears and they were ok but the 10-17's are better and found in the 71 to 79's 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.:thumbsup:
 

·
Supporting Vendor
Joined
·
3,770 Posts
Discussion Starter #2 (Edited)
POSI TUNING Pt 2

Ok, I'm back:laughing:

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 the 4 springs and plates these were built with. 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:rolling: 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. All this did was to weaken the clutches and they still would hammer only now they would break.

Here is a 74 posi I rebuilt. 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. I guess the theory is the huge springs will cause the fiber to slip when loaded in corners. This will help with chattering but the material is being wore off, what happens when the fiber is gone and where did it go- into the oil.

Here is a new fiber clutch



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



I use solid clutches only. NOTE: 40% price increase on 1/1/15





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" I have seen the new GM supplied Eaton cases have a much tighter tolerance and the shims are usually within 002" per side. The older cases vary a lot, I have tuned them and had 010" difference between them.




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 assembled 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. The posi has to be a balanced unit, the cross shaft should slide easily out when the posi is held in any position. If you have the posi on the axle on the LH side and the cross shaft binds up then flip it over to the RH side and it's free then it is not balanced and is not going to work. You have to seat the clutched by rotating the assembly on both side, CW and CCW 50 turns each. I end up around 500 or more.
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.:smack



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

·
Registered
Joined
·
1,079 Posts
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

:cheers:


 

·
Premium Member
Joined
·
10,260 Posts
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.:thumbsup:
happy to help, Gary. This is exactly what I'll be needing next winter when I tear into the rearend. :partyon:
 

·
Supporting Vendor
Joined
·
3,770 Posts
Discussion Starter #5
Thanks Jason, I knew you would fix it up:thumbsup: If you get into a hassle with yours let me know and I'll walk you through the job.
 

·
Supporting Vendor
Joined
·
3,770 Posts
Discussion Starter #7
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.

 

·
Registered
Joined
·
1,497 Posts
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
 

·
Supporting Vendor
Joined
·
3,770 Posts
Discussion Starter #9
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.
 

·
Registered
Joined
·
1,497 Posts
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
 

·
Supporting Vendor
Joined
·
3,770 Posts
Discussion Starter #11
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.
 

·
Registered
Joined
·
552 Posts
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.
 

·
Supporting Vendor
Joined
·
3,770 Posts
Discussion Starter #13
FYI
If you haven't seen it, Tom has posi tuning on You tube.
 

·
Registered
Joined
·
110 Posts
those are good videos! thanks for the tip-off. didn't think that Tom's Differentials had embraced the information age...
 

·
Supporting Vendor
Joined
·
3,770 Posts
Discussion Starter #15 (Edited)
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.

 

·
Registered
Joined
·
1,071 Posts
Sometimes I am ALMOST impressed with your tools as I am with your work!!!:laughing:
American Ingenuity at its BEST!!!:cheers:
 

·
Supporting Vendor
Joined
·
3,770 Posts
Discussion Starter #17
Hi Guys,
Just a note. I have heard from some guys recently who have given posi tuning a go and had some issues.

So to recap, this does work BUT you have to follow all the steps, skip one and it's not going to work. That is why production built units have the springs. You can have a wide range of lash and use the springs to make up the difference.

So tune a posi:
1- Do a good polishing job. The ones I have seen posted online were not complete. I would never ship out what I have seen posted. You need to do more then lightly dress the windows. Fail here and cracks can form as before.

2- Use the solid steels. I edited this post and noted there was a huge price increase on these at the beginning of the year. 40% to be exact. Hopefully that doesn't mean they are soon to be discontinued.

3- You may need to have shims ground in between the 005 increment sizes the shims come in. If you can't find a shop to grind them let me know and I can supply ground shims for you.

4- The unit must be balanced. This means the cross shaft slides freely in and out of the case with the case in any position. If you don't get this correct the posi is not going to work right.

5 - Seat the clutches. This is very important. I put in about 500-800 revs per side, cw & ccw to seat them by the time I'm done. Then and only then can you correctly shim the posi correctly. Do at least 50 rev's in all directions before making a shim change. I do them on my machine and run them in 300 rev's before a shim change BUT I did plenty by hand and they can be done that way. The machine works the best.

If you find the posi shifting under load it is not correctly setup. Once correctly setup, there is no lash and no movement in the gears, yet it will rotate fine. Look over the Tom's video again. Once tuned you do not need the springs. I have them in street, street/strip, road race and the rarest of C2's. No bragging here - just fact.
 

·
Registered
Joined
·
350 Posts
Thanks for the update. Good timing too.
I was just looking up this thread to go through a Chevy 10 bolt 8.2 this weekend.
 

·
Supporting Vendor
Joined
·
3,770 Posts
Discussion Starter #20
Well today was the first day to hit 60*f in about 5-6 months so I took the car cover off, pulled the trickle charger, checked under the hood and fired it up for a 20 minute warm up and ride.

The tuned posi work flawlessly, no shifting, chattering, nothing, yet still has full posi.

You guys can do the same just take your time, differential work is not a fast process- if you want to do it right.
 
1 - 20 of 29 Posts
Top