Rebuilding a 63-79 Differential?

[Gary R.]

Re: Rebuilding a 63-79 Differential?

Thanks Mike, I never used that but it make sense.Many times I need to dial them in by grinding anyway but that chart is good. I have seen some with the size etched into them too, not too many, I think they were in the 064 range.

[Michael H.]

Re: Rebuilding a 63-79 Differential?

The narrows used a cast ground shim, I've never seen an original factory setup with multiple stacked shims.

Factory assembled diff's never used the stamped steel selective shims, as far as I know. All were the ground cast spacer that you mentioned.

For service, there was a special spacer ring that was to be used with one selective shim. If you see these steel rings along with one shim, it's likely the unit was rebuilt at a Chev dealer where the rings would be available.
Most non dealer shops just used a stack of selective shims instead.

I suppose it was much easier to stock the spacer ring and an assortment of thin shims than it would have been to stock a wide range of cast/ground spacers.

[Gary R.]

Re: Rebuilding a 63-79 Differential?

Also I mixed up that statement, the narrowes used the thin "STEEL" shims, the wider housings used the single cast,ground shim.

[Joe C.]

Re: Rebuilding a 63-79 Differential?

Hi Gary,

Your work is impressive, and not that I consider myself experienced in differentials, but I have never seen a polished carrier before. I assume that it cuts churning of the diff lube, keeps things somewhat cooler in there, and reduces power loss through the diff. If this is the case, then have you found any measurable or apparent difference by doing this?

I rebuilt my Eaton posi unit, and I seem to remember that the test rotating torque for the clutches was specified at no less than 40 ft-lbs. I measured mine at 59 ft-lbs at 70 degrees ambient temp. What can you tell me about that.

Thanks,
Joe

[Gary R.]

Re: Rebuilding a 63-79 Differential?

Hey Joe,
Polishing and tuning a posi is something that started, to my knowledge, back in the late 60's in the greater LA area among the hot rodders of the day, Pepe Estrada and Tom Watt in particular. Tom is the owner of Tom's Differentials, now in ID.

I learned it from Tom and added some of my own mods to the process.
The reason for polishing it is to cut down the potential for cracks forming and wrecking the posi case. I have many pictures of this process on Digital Corvettes you can check if you like.

Tuning a posi eliminates the plates and springs stock units have. I've heard all the arguments that this will not work or the clutches will wear out and usually those making that statement have never built or properly built a polished or tuned posi. Once tuned, the spec on 40+ ft/lb break away torque will not apply. You can easily rotate one axle against the other by hand. Again properly setup the posi will lock up but not have the common clutch chatter associated with the springs and plates. The clutches will not wear out faster because there is no added load on them from the springs.

I have to admit I was at first skepical of this at first but it was on my own 69 so if it didn't work like I wanted I would have removed it. It worked great. Then I setup one for my son's 75 and told him to have at it and trash it. I even started with a scored case to test it. After 8,000 very hard miles of 200' burnout, launches, and about every type of abuse thrown at it, the posi blew up. Granted this is not the type of driving most vette owners,myself included, will put their cars through but it was a good test to see for myself. Tom's diff's have been at the tracks since 1975 with passes in the 8 sec range I believe. Stock vette diffs will not hold up to this abuse. After my son took out the test case, I built him a full 12 bolt conversion with 3.5 1/2 shafts and 31 spline outer axles. He then blew up the ST-10 trans!

[Michael H.]

Re: Rebuilding a 63-79 Differential?

Hey Joe,
Polishing and tuning a posi is something that started, to my knowledge, back in the late 60's !

Yup, you are correct, Gary. It may have been GM that started it. In 1967-69, for Trans-Am Camaro racing, GM recommended that the edges of "window area", or opening in the diff case, be ground smooth and shot peened. The process would be similar to the process used for connecting rods.

I'm pretty sure I still have the sheet that GM sent at that time that describes the operation. If I can find it, I'll scan/post it.

There was also a HD 22 plate positraction unit available in 68-69.

[Joe C.]

Re: Rebuilding a 63-79 Differential?

The posi unit I rebuilt for myself was a few years ago, and (I think) the reason the rotational torque was 59 ft-lbs (actually, BREAKAWAY torque is momentarily much higher............once the clutches are slipping, the steady state torque drops to the rotational value) is because I set it up using the minimum recommended endplay of .005 (I like to tweak shims by manually adjusting their thickness in order to get tolerances exactly where I want them).

[Wayne M.]

Re: Rebuilding a 63-79 Differential?

Wayne ....Ok so here is a test for you.
What is this one I have here ?

Gary -- I'll guess at the wide(r) carrier, mainly by those 3 tits, which match the 3899143 of April '67 that I show below (thumbnail).

[Gary R.]

Re: Rebuilding a 63-79 Differential?

Sorry Wayne, it's a narrow carrier. The steel cap isn't a stock one . Look behind the cap at the inside edge of the carrier and you'll see it's a straight edge no step.

This picture is better, it's a super 10. Look at the inner edge and compare you'll see the jog or step cast into it. These use the approx .250 thick shims.

[Rich W.]

Re: Rebuilding a 63-79 Differential?

Hi Gary..I remember reading this a little while ago and figured you should have an answer for me..I am doing the TAs on a 73 and the Ps TA front bushing was literally gone..it was beating the suspension to death..anyhow when I removed the TA assembly I noticed I could pull out and in on the driveshaft about a quarter of an inch..the other side barely moves..is this usually an output shaft that is damaged or does the diff need a complete go thru?I figured I am a good bit there in the removal process so its not a big deal to keep going..just looking for some diag pointers if you could throw some my way..

Thanks a bunch

[Gary R.]

Re: Rebuilding a 63-79 Differential?

Hi Rich,
First on the T/A setup. Read over my thread on rebuilding them and try to setup the endplay at 002. I go to 0015 but I can grind the shims to size to dial them in, most guys can't. Also watch the bushings you buy, I've seen a lot of junk being sold these days and some new rubber bushings have issues like the sleeve ID being to small, flare ends cracking,and some of them appear to be plated and don't fit well in the arm. Use ONLY USA Timken bearings if you want the best. The cost of bearings is going up again next month from what I was told and I've seen cheaper substitutes in diff kits.

For the diff, I'm assuming you're talking about the axles(yokes) as opposed to the pinion yoke. If the pinon yoke is moving in/out then you have some issues as a .250 movement will hit the posi case and gouge it pretty good and it will sound very loud when you let off on the gas.

Now the side yokes are probably what you're referring to. 73 yokes are near the time the quality on them went out the door. I have reused some 73 yokes but I suspect you could have a couple of issues. One is the snap ring,(Circlip), came off. This isn't too common but sometimes happens. Usually excessive sideplay, and this term is open to interpretation, I call anything over 015 excessive, is caused be poor posi setup, worn yoke face, or loose case side hole. On an otherwise good diff up to 045-050 endplay can be used.

Since you have the car down, your best bet is to drop the diff and pull the cover for a look. You can see the yoke ends, check the posi case for cracks and drain the oil for a PM. Me I would be replacing those snowflake clutches with solid steels but thats' your call.

[Rich W.]

Re: Rebuilding a 63-79 Differential?

Thanks for the fast reply..the TA bearing setup you nailed on the head..I only use the Timken bearings and I do what I can to get them tight..I have a friend with a mill that can machine the shims to whatever thickness I need! Makes life nice..my problem is the side yokes..I haven't really tried to pull it out yet to see..but I think your right on the pull it and see what's going on in there..may need some help on parts!

Thanks again..

[Rich W.]

Re: Rebuilding a 63-79 Differential?

Got the whole thing out this afternoon..some of the bolts were allready messed up so I am taking it to work to cut the rest apart with a torch..then I can really look at the diff and see why it is so loose..the clips are in there fine..the yokes just move in and out ..I will measure it also tomorrow since I have those tools in my box at work also..