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Old 06-04-2007, 04:46 PM   #1
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Differential Rebuilding, '63-'79 Iron Units

This is going to be a long post. I had one running elsewhere for years now and it grew into a very long post. I'll try to gather pictures of several past jobs to show you the differences and options you can do or have done.

Differentials have been a mysterious unit for most corvette owners for a long time. Replacing or building an engine no problem but digging into a diff to make it stronger that's another story. Going beyond changing the oil is not where most owners want to go. Thinking a shop will do what you want or expect can be a very expensive disappointment.

If you follow this thread along with the GM overhaul manual of the 60's - early 70's it will become more clear.

The point of this and all my threads is the education of the corvette owner so they can make a sound decision as to if they want to tackle this job. You must be honest with yourself - do you have the tools, measurement tools, the time and a good place to work? Cost wise you'll save $500-$600 in labor but the big cost are the parts. I use the best and they cost the most. I suggest you use the best parts as well but that is your choice. If you farm it out ask what make the parts are before you commit. They are not all the same.

The 63 differential is different then the 64-79's. the cover uses a different fill plug, the bearing caps are not as strong, the posi cases were not the Eaton brand as used in the 65-79's. Look up my posi tuning thread for more info on posi's. I will not go into great detail on posi's in this thread. Once I post the pictures I will limit the text in order not to write a book here.

Lastly, this is the way I do the job. I did not invent it, just reworked it. There are many who do this, some may agree or disagree. I don't care, I have yet to see anyone else post this type of information- correctly.

If you have any doubts about your abilities to do this job, pay someone who knows what they're doing. Remember, I am not responsible for your work.


Ok, some of you may recognize these pictures.

Here is a typical stock corvette differential. Out of the car and mounted on an engine stand. This really works great to overhaul these and the IRS pumkin is small enough to work around.



This is what I use to remove them with the car on jack stands. Works great.


Here it is with the cover off. The early units use 5 line bolts not the GR8 6 lines, and the bearing caps don't have the cast in tabs like the one on the RH side here


You hear a lot about 3 series and 4 series cases. The 3 series is the common one and the one you want. You can run 273-433 gears on it. The 4 series is a bit of a dinasour now as it was used with GM 411, 456, and up gears.
Here they are. The 4 series has a thicker flange to mount the ring gear.In this picture the 4 series has a 411 gear on it.



For comparision here is an 80-82 Differential. This is my only reference to them.




With the cover off take a look at it. They fail in the spring mounting ears and internally if someone used a bolt that was too long. They should not be cracked, leak, or show signs of a ring gear bolt flying around inside it! This one had the casting broken from a long bolt. The owner didn't know it as it was an eBay purchase. Be careful if you think buying a used diff is the answer.



The early housings, 63-66 or 67?, were narrower between the bearing pads. Look in any catalog and they list master kits for '65-79, when the Eaton posi's were used. The shims in the common kits are not the same as the original thin shims and will not work- although some may try. You need to grind shims for the early housing to correctly dial in the backlash.

Here is what I'm saying.

63 housing




'69 housing




Here are the shims from the kit. They are stackable for ease in changing backlash setups. They run about .250" +/-.020"



The early shims are in the .070"-.090" range and need to be ground to size on a surface grinder. Setting up an early diff,correctly, takes more time to do. Without access to a grinder you'll find all kinds of bubba setups.



Here is a thin shim in place, you can see it is about .080"




Before removing the case, witness the caps. Here is one I stamped an R into.The cap screws are also a mod I do.




Look over the housing. Here is one I got in from FL. The front bolt was rust welded to the housing. I had to cut the bolt head off to remove the bracket. This bolt was really stuck in there. I had to cherry it with an oxy-acy torch to break it loose. Who said FL cars don't rust?




Here it is after I got it out.



One of the things I do is grind the sharp edges. Helps to prevent cracks and keeps your hands in bette shape







I also tap a drain hole in the housing if the owner wants it. This is done at NC, unlike some places that I've heard of charging up to $50. I tap them 1/4 npt and fit a brass plug. You cna use a magnetic plug as well but I like the brass. For aluminun tranny's I tap them 1/4 npt then use a 1/4 x1/8 bushing and 1/8 plug. This way you don't strip out the hole in the casting when wrenching on it. Here is one in an iron diff.



But this is all you need for these. If you have not tapped a pipe thread they are tapered so do not bottom out the tap like you would a machine thread or the plug will not seal. Tap and fit until you have about 1/2 the threads on the plug showing.





END OF PART 1

Last edited by gtr1999; 01-23-2012 at 03:31 PM.
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Old 06-04-2007, 06:00 PM   #2
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Differential Rebuilding, '63-'79 Iron Units Pt2

So now we have an idea of what in there. You know the differences in the housing and cases. The posi cases setup with the 4 springs to take up any tolerance loss during assembly. They provide posi action but if too tight then the diff will hammer around corners. Look at the posi thread you'll see the clutch types and recommendations. No sense going over that again here.

You have your posi built, ready to install the new ring gear. Now if you are planning on re-using your old gears and they didn't make noise and there is no sign of damage you need to get them back to where they were. You have to take notes on the backlash and pattern BEFORE you take the unit apart. If you don't get the old gears back to where they were seated in they'll make noise forever. I prefer to install new gears, everything else is new why not use a quality gear set from Toms?

I NEVER reuse ring gear bolts, unlike some jobs that I've seen. To mount the ring gear I cut the heads off 1/2 of the old RG bolts and slot them. Then screw them into the RG and they will align the gear as it is pressed on or drawn up on the case.







Use Loctite #271 to hold the bolts in place and torque to 60 ft/lbs. Clean the htreads with brake cleaner and the RG holes. The RG and case flange surfaces should always be stoned prior to RG install. When I have the case apart to polish it I also polish the flange in the lathe.






The posi case and RG are now ready to install and begin setup. So now we'll look at the pinion setup. The pinion bearings are preloaded by a "crush sleeve" with the yoke,washer and nut in place the nut is set to produce 15-18 in/lbs of drag. This is done in setup without the seal or sleeve. Just install the bearings,(I use setup bearings to start),yoke, washer and the old nut. take the old nut and grind off the lock ring portion so you have a plain nut to use. There is a shim under the large pinion bearing. This bearing is a press fit so you will need a bearing splitter and press to remove it. Start with the original shim. Mic it up and record the size. I always use a set up sheet to track the job as you can get confused quickly.











Here are some sleeves. Original for reference, new stock type and a solid sleeve that is fit like the rear bearings are. The solids are nice once setup but these also require machine work. The shims in the kit do not offer enough range to dial in 15-18 in/lbs.



Here is what the pinion looks like in the housing. The sleeve is a solid one but the crush sleeve fits in the same way.



The setup bearings I start with are Timkens that I polished the ID's on to give me a slip fit. That way I don't have to press the new bearings on/off to dial in the pattern. Setting up the pattern is a time consuming job. Do not start this part until you are ready.

Ok so with the old shim on the new pinion, setup bearings, install the pinon into the housing. The old races should have been removed during tear down and new races should be in place. Oil the bearing with 90 wt gear oil. No sleeve, no seal, add the outer bearing, yoke, washer and nut. Tighen until you have 15-18 in/lb drag. You have to use a 0-30 in/lb dial wrench. Using a beam or clicker just isn't going to work.



This is a puller I use to remove the yoke during tear down or the many times you have to change the pinion shim during pattern setup.




Ok now before you can drop the case in you have to install the new bearings. You are using Timkens, right? The kits I sell have Timkens but you should check. Again some may not agree but this isn't their thread!


These bearings sit lower then the hub of the case. You have to press them on fully. I made some tools to do this. You'll see what I mean below.








Be careful you do not damage the bearings when you press them on.

Now you have the case done, bearings installed, pinion in place drag set. Install the case and races in the housing. Be careful and don't drop the case in or you'll damage the new gears you just bought. You can see from the setup sheet how I played with the shims to get the backlash close. I now set Tom's gears to .005-.006 backlash. There is a coating on them that will wear off and open the BL up .001-.0015" so 5-6 is good.
Be sure the bearing caps are installed and torqued to 55-60 ft/lbs before checking the backlash. With the later housings you will be able to get the BL close, with the early units you may need to grind shims to get to a starting points and will contunue to grind them until the pattern is correct.

Once the backlash is good you can test the pattern. Using the yellow paste in my kit or buying it from any GM dealership, coat 3-4 teeth. I do it in 2 places. You them need to load the gears while spinning the ring gear in one direction a few turns then in the opposite direction. I use an old leather belt on the larger pinioin yokes to apply drag and this fixture I made on the smaller pininon yokes.




Take a look at your pattern to see where the pinion is in relation to the ring gear center line. You want the pattern between the face and flank to be equal. This is the top and bottom length of the tooth. The end of the tooth to the center is called the toe and the end to the OD of the gear is the heel. The concave side of the tooth is the coast and the convex is the drive.


Here is a bad pattern. The root is longer then the top of the tooth. This is the drive side and it is to the toe.



Go online to Randy's Ring and Pinion and you'll find installation info with a lot of pattens. Changing both the backlash and pinion shim will move the pattern around. You want the 5-6 BL, sometimes this will open to 5-8 or even 9. Most times it is within .002-.003". The pinion shim is where you'll make the changes untill you get a good pattern. Then you'll remove the setup bearings and install the new lower bearing and recheck. Don't be surprised if the pattern is off a little. You may have to change the shim a couple of more times until you get the pattern with the new bearings.


Here are the final patterns on a set of 373's I built for a BB crate car. The guy loves them. You can see they are to the toe where performance set tend to be, under load the pattern will spread out to the heel.





Well that's it for tonight. The next part will cover pinion seal install, yoke fit, and final assembly. Then I'll do one on the options you can add like a solid sleeve, steel cap, and ARP RG Bolts.
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Old 06-05-2007, 11:41 AM   #3
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Differential Rebuilding, '63-'79 Iron Units Pt3

PART 3

Ok now we can look at installing the yoke bearings. You don't need them installed when doing a pattern check. I use Torrington full compliment bearings. The yoke bearings in some kits are caged with 1/2 the rollers, some kits don't even have the yoke bearings in them.
Here are the 2. There is debate over which is the correct one to use and I'm not going there. I use the Torrington's unless someone wants to save $20.



When I tune a posi I usually have to grind the face of the yokes to fit. The endplay can be between 0-.005 before grinding and I like to set them to .005-.010" I save the yoke install to last. Once they are fit I install the seals.

With the pattern correct with the new bearings, I final set them. This is when you install the crush sleeve or fit the solid sleeve, install the new seal and new nut. I check the depth of the bore for the seal to be sure it seats and that the flange does not hit the yoke sheild.











Once set I grease up the seal and use RTV on the OD and set it. There usually is about a .125 gap between the housing and flange. If you hammer it home,past the counterbore it may collapse the seal and it will leak.






Don't for get to use #2 on the splines or it will leak for sure.



If the yoke OD is scored or grooved then you need a special speedy sleeve. This are not cheap and may not be available locally but they work.









Final torque check.





With the pinion done I go back and setup the final case shims, only .005 over per side to preload them. If this is an early housing that means grinding 2 more shims to size. Do one last pattern test, install the yokes and fit and you're done. This one is a HD unit I built- notice the LH steel cap.








The next and last part will cover the HP things you can do to these iron units.
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Old 06-05-2007, 04:17 PM   #4
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Differential Rebuilding, '63-'79 Iron Units Pt4

OK this is the end of the show. If you followed the GM overhaul manual, this thread, maybe the old one I have on CF, and if you are lucky to have a copy of my old paper you're ready to do the job, ....right?

Now if you want to beef it up better then stock and unlike all the exchange units on the market, you can do the following.

1- Polish and Tune the posi- we already covered this but this is the #1 mod I would do. I have never had one yet that hammers or chatters.

2- Install socket head cap screws in place of the hex head bearing cap bolts.

3- Fit a steel cap to the left side. Takes some work, but adds strength if you're going to hammer it.

4- Fit a solid sleeve in place of the crush sleeve. Holds it solid and you'll be able to replace the pinion seal in the future much easier. Sooner or later the pinion seal will leak, they all do. Adding grease and the speedy sleeve to the yoke will help as does the #2 on the splines.

5- Install ARP ring gear bolts. These are rated higher then the grade 8's in the kits. The kit bolts aren't bad but the ARP's have a shoulder like the originals. They are used without washers so you have to use #271 red Loctite and chamfer the case holes to fit them.

6- You can add a HD cover as well

7- Billet pinion yokes are also available but I haven't used many.

The next step would be upgrading to a 12 bolt and HD yokes,unless you're going to drag race the car this would be the ultimate setup, while still maintaining the IRS. Some of you have gone to 9" but that is a whole different story and I'll leave that for the guys who have done it, I haven't.

I think this is the last of my pictorials for you folks. I hope you have made use of them and they gave you something to think about when you need to address any of the topics I covered.



Chamfer holes for ARP bolts




ARP's installed and close to the housing.



Ground heads - just because I didn't like them that close to the housing.



Norvals solid sleeve- custom machined to perfection



The solid sleeve I use- has to be fit with a surface grinder so add more time to the job.




Steel and stock caps





These too have to be fit and I do it in a Bridgeport Mill. Indicate it across the pads, drill the 7/16 -14 holes out and tap them for 1/2-13's.






Here is the final product with steel cap, bolts, solid sleeve, polished and tuned posi, new Toms gears, 10-17's, the whole 10-bolt ball of wax.These are the same as above, they must have been mixed in transfer.






As Tom says in his catalog "They're fun- try building one" He's a cool guy.

Good Night Irene!!!

Last edited by gtr1999; 06-06-2007 at 05:13 PM.
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Old 06-08-2007, 07:15 AM   #5
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NOTE: when using the yellow marking paste I have in my kits or the GM stuff, put it in a small plastic container- little bigger then a pill case is good or a spray can top. It is usually a little thick, so add a drop or two of gear oil and mix it up, just don't add too much or it will run and will not give a good reading.
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Old 11-06-2007, 12:11 PM   #6
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One thing I might have missed out posting, is to check the yokes when you get them out. A lot of times a diff may have .030-.040 endplay and it's not the yokes but the posi play. Again I didn't reread this post but have been seeing this question lately.
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Old 11-06-2007, 12:58 PM   #7
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Once again, Gary, GREAT Post..

All we need now is one of the MODS to move it up to a sticky!!


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Horsepower is like potato chips. If you have 300, you want 500. If you have 500, you want 800. One is never enough.
Old 11-06-2007, 03:53 PM   #8
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Tim this is under the rear end/suspension sticky. There are a few topics there. I just add to them as I come across things that are new or may have been left out.
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Old 11-16-2007, 12:08 PM   #9
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I posted this under the yoke inspection post.

If you buy the rebuilt yokes, and those are what I use, look them over when you get them. I had some come in that were not rebuilt correctly and the splines didn't line up.



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Old 11-16-2007, 02:35 PM   #10
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Did these come from Tom's ?
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Old 01-20-2008, 10:59 AM   #11
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There always seem to be questions on stamping and codes on the diff.
I really pay too much attention to them as you never now what may have been done to them over 40 years.
Here is a housing I just finished spending a lot of time cleaning. It's out of a 1973.




Here;s the rear code, It should be a 336 ratio but I didn't look up the numbers.



Here is the casting code, upside down on the LH side. D18 3, (April 18, 1973)


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Old 01-20-2008, 12:13 PM   #12
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Great post Gary. Thanks for the effort. Very timely for me.
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Old 01-21-2008, 07:41 PM   #13
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Thanks for the date info Gary. What do you use to clean and paint the housings? That looks great.
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Old 01-22-2008, 06:41 AM   #14
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It takes me a while to get them clean. First I scrape off the dirt and grease, then take it apart,wash it in my cleaning tank to remove the oil inside, then blast it in my cabinet, scrape off the left over heavy stuff like undercoating, blast it again, and in this case wire brush it. Then wash it again, POR15 etch it, POR15 paint and top coat with seymour high temp.

Here is the one I'm working on that you mentioned.

As it came in




ready for Etch or left natural for NCRS





Here is one I did a while ago but same process with the POR15



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Old 02-01-2008, 09:03 AM   #15
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I get asked a lot about whose gears I use. Most of you know the brand.

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