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Old 11-20-2016, 09:14 AM   #1
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Winter Differential Rebuild Project

So I have a partially unwarranted need to change gears on my 72. Currently has a set of 3.08 but the latest version of the engine (454/auto) isn't happy idling around town at basically idle.
I've gone through various scenarios and 3.55 would seem to be the correct solution with a possibility of changing to a OD trans down the road somewhere but sticking with the TH400 for now.

I know sending it out (you all know who you are) is the safest way to have it done but I'd like to have a go at it myself, especially since I need a project for this winter.

I'm making this thread with the hopes that the resident expert(s) keep an eye on it and point out where and when I go wrong

At the end I will not a drag rear or anything but hopefully a functioning street diff that I won't have to worry about (too much)

Who knows, maybe won't even make it past opening up the case....

Last edited by Mooser; 11-20-2016 at 09:47 AM.
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Old 11-20-2016, 09:38 AM   #2
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Here's what hopefully will be the patient. Don't know the history and other than the diff. turns smoothly and it still has the rivets in the rotors... I've got nothing








What's visible on the stamp matches the gears (turning and counting) 3.08


Can't get a year off it yet but AW didn't go past 75 as far as I can find

M
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Old 11-20-2016, 09:39 AM   #3
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whats the rear tire diameter?
as a general rule youll want the transmission first gear . multiplied times the rear gear ratio to fall in the 10:1-10.6:1 range

http://garage.grumpysperformance.com...44/#post-53346

http://garage.grumpysperformance.com...ine-combo.741/

http://garage.grumpysperformance.com...ift-calcs.555/
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IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
 
Old 11-20-2016, 10:09 AM   #4
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The tires are (currently 235 / 70R15) and stand at about 26-1/2 ~ 27" tall, calculates at about 27-1/2 ~ 28.

Problem I'm into is around town it's trying to run at 1400-1500 RPM, at / below the bottom of the cam (depending on what chart I'm using 1600 or 1800 is the low end) and the car (carb) surges just enough to be annoying (under load it's OK but crawling around town just not quite happy)

Remainder of the time around here it's 1900+ RPM and on the highway 2600 RPM

I'm about 30% in town and 70% between towns / highway so don't want to go up too far, OD trans may never happen or at least not anytime soon.
M
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Old 11-20-2016, 10:32 AM   #5
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No loose bits


Or any other obvious signs of carnage


Clips are there and there's something left on the ends


.039" and .049" endplay on the yokes vs cross-shaft

There is some backlash between the pinion and ring but no noticeable end-play on the pinion

Everything is draining now, I'll stamp the caps before I forget and see if I can get the yokes to come out.

I'd like to figure out what I need to order (looks like Toms) before the Canadian Peso falls any farther

M

Last edited by Mooser; 11-20-2016 at 11:33 AM.
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Old 11-20-2016, 01:11 PM   #6
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Damn
One yoke is .138" end to the groove, the other is .170". Worn more that I was hoping.



Also found 2 bolts on the ring gear that are different than all the rest... Someone's been here before or just the luck of the draw that day during assy.





Pinon gear is stamped 12 37 5 73

so maybe a rear out of a 73?
M

Last edited by Mooser; 11-20-2016 at 04:50 PM.
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Old 11-20-2016, 03:30 PM   #7
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Someones definitely been there before, OE pinion typically has no stamping. At least that is what I understand and what I have seen.

Glad to see someone else doing this. Not that hard, but tons of time to get it right.
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Old 11-20-2016, 04:41 PM   #8
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Sometimes I wonder if I shouldn't be more intimidated by jobs like these but where's the fun in that?

I've found a few images where the front of the pinions are stamped, some from C2s and some from later C3s so I'm not sure what the story is there. Other than the two odd bolts the thing certainly didn't look like anyone had ever been near any of it. Who knows.

I'm going to pull the pinion tomorrow and pull apart the posi unit, wash everything down and see if there's any issues with cracks or marks or anything.

I'll have to decide what to do about the side yokes, could hardsurface and grind them here, use them as cores for a set of "re-manufactured" ones.
I wonder what's the story with the O1 buttons they were using for awhile...

M
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Old 11-20-2016, 04:50 PM   #9
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I had the same issue, maybe slightly worse. I just replaced them with a set from Tom's. Not that expensive really and better to do it now than have to dig in there again in the future.

A good time for 30 splines and not that hard to install if you have a way of boring the posi case to accept. Although with 30 splines you need to replace the spiders, etc. as well. Again, probably not a bad idea while you are there.

If not a new set of 17 splines will do the trick.
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Old 11-20-2016, 04:54 PM   #10
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Quote:
Originally Posted by Mooser View Post
Sometimes I wonder if I shouldn't be more intimidated by jobs like these but where's the fun in that?
These jobs, while intimidating, are all just one step at a time. May have to redo some stuff, maybe buy a tool or two, but it is going to be cheaper if you do it yourself or worse case the same amount if you have to purchase a tool. Then you have a tool for the next job.

I prefer to do it myself. Then I know what is right and what maybe wrong.
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Old 11-20-2016, 05:39 PM   #11
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There were a couple of suppliers of ring gear bolts, it is possible they are original. Typically they would be replaced if they backed out and sheared. You would see damage to the web side of the posi case and the LH interior wall if a bolt backed out.

Looks like a typical original 74 308 setup out of a automatic.

Looking at your pictures I could clearly see the ends of the yoke worn and rolled back- look at the spline ends. Unworn ends are smooth and chamfered.

Those are original GM gears, they were date coded and ratio stamped. May 1973 308's. Look on the side of the ring gear for the same thing. There is a GM stamped in there, that is how they all came. Aftermarket gears were not stamped like that.

I can see the cast side shims in place so I doubt this was ever overhauled, other clues are the bearings. If they are original they will NDH- the only bearings used by the factory, anything else including Timken are replacements.

Look at the caps and side face to see if there are any witness marks on them. Factory caps might have paint on them but no stampings or punch marks.

If you are going to use new gears use ONLY Tom's, the last usa made gears left for a vette. 73vettes were still 5 cut gears and the 355's I am going to setup next week are also 5 cut gears. 2 cut are not tapered from the toe to heel.

The exterior of the housing looks scaly so you want to make sure it's not thin at the radius's I would doubt it but you want to be sure.

So you're going to do it yourself, good deal. You could go local, to a vendor, to the guy on CF who will try and sell you on things you don't need, or use a guy like me~ ! LOL. I will build it better then anyone but you can come pretty close if you take the time and do it right. You did ok on the box but you're going to have to do better with the diff, just being direct here so you know what you're getting into. I will be busy so I don't know how much I'll be on here from time to time. If you're going to ask the CF knock off artist for help then I won't respond beyond this post.

You're going to need an engine stand, 0-30 in/lb DIAL torque wrench, a good indicator- I like Starrett 0-1". access to a surface grinder, Bridgeport, lathe, and press - depending on how you plan to build the diff. As 73 said the 30 spline builds are nice but the good spiders are gone now, the replacement are not as good as what Tom had for years. I still have some but those are going into full super10 or 12 bolt builds in the future. 17 spline axles- use rebuilt over new. The QC on them is not what it should be and never has been but still better then the new imported axles.

You can put it together like a vendor but if you're going to do it why not build it better? Look over my threads and Tom's you tube videos on tuning the posi. Blend and polish the posi real good inside and out. Check the run out in the flange with this one since the RG possibly could have been replaced.

Parts
USA Timken bearings- only. CR pinion seal only
ARP ring bolts
Solid Steel clutches only
New cross shaft- mill case pad deeper for better support
Magnetic drain plug
Socket heads for the caps, make sure they are fit to 001"
Set axle end play to 005-008"

A proper setup blueprinted diff should be good into the 400hp range but ratings are very subjective to use, power, trans, traction, and usage.

A super 10- a real one not the knock off artist version, is good into the 550hp range but I have them in cars up to 700hp. Not what I recommend but it was the owners choice.

Over 550hp I recommend a 12 bolt, 1480 being the best.

So you go, I would blast, etch, POR15 and top coat the housing and cover. Powder coating is not worth it and a plain paint job will rust in a year or two if you drive the car.

It takes me about 20 - 25 hours to build one of these with a good 5-6 hours into the posi alone and I have done 100's of them.

Good luck, you'll do ok just don't cut corners.
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Gary
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Old 11-20-2016, 06:19 PM   #12
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Quote:
Originally Posted by gtr1999 View Post

Looking at your pictures I could clearly see the ends of the yoke worn and rolled back...

I can see the cast side shims in place so...
If they are original they will NDH...

Look at the caps and side face to see if there are any witness marks on them. Factory caps might have paint on them but no stampings or punch marks.

If you are going to use new gears use ONLY Tom's

The exterior of the housing looks scaly...

So you're going to do it yourself, good deal...
I will build it better then anyone but you can come pretty close if you take the time and do it right.

You did ok on the box but you're going to have to do better with the diff, just being direct here so you know what you're getting into.
I will be busy so I don't know how much I'll be on here from time to time. If you're going to ask the CF knock off artist for help then I won't respond beyond this post.


You're going to need an engine stand, 0-30 in/lb DIAL torque wrench, a good indicator- I like Starrett 0-1". access to a surface grinder, Bridgeport, lathe, and press - depending on how you plan to build the diff...

...17 spline axles- use rebuilt over new. The QC on them is not what it should be and never has been but still better then the new imported axles.

Look over my threads and Tom's you tube videos on tuning the posi. Blend and polish the posi real good inside and out. Check the run out in the flange with this one since the RG possibly could have been replaced.

So you go, I would blast, etch, POR15 and top coat the housing and cover...

Good luck, you'll do ok just don't cut corners.
Thanks Gary
Yes the chamfers on the splines are pretty much gone, the rolled burr was very light and the yokes came out with a few taps.
BRGs are the NDH and the cast spacers were on both side with no other shims
No ID marks on either of the two caps, top front, sides etc. (well there are now )

I'm trying to get everything I need to go to the USA for through Toms if I can, some of the items (ie Timkins) I can get here (Ontario) easily, but the specialty (gears, shims, clutches) all come from down there anyway so one shipment across the boarder is easier and will only hurt once.

Housing isn't quite as bad as it looks in the pics but I'll be going over the entire thing inside and out anyway so I'll watch for anything that looks off.

I'll be finishing it as you said, POR15 and then a top coat. The rear that's in the car (done years ago) got a "mop & glo rebuild" when I had the rear suspension out a while back. It works perfectly so I'm hoping to leave it alone until I need to steal the HD cover off it and by that time I'll know this is ready to go.


I've got most of the equipment & tools here (lathe, mills, press, etc) and still have access to anything I might not have (surface grinders, hones, etc)

I'll stick with the 17 spline axles, will suit my needs just fine and hopefully I can find a source for rebuilt (didn't see them in Toms cat. but maybe I missed it)

I've watched all the threads, stickies, videos, copied and printed many sheets with hints tips and look-out-for stuff. Once I get this cleaned off and know what I need to order it'll go to the side and be one of those things I can work on when I'm feeling like it rather than a need it for such and such a date.
But still hopefully have it for next season

Thanks again
M

Last edited by Mooser; 12-21-2016 at 09:03 PM.
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Old 11-20-2016, 06:50 PM   #13
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Sounds like you have it covered pretty good. With access to the machines you can build a very reliable and strong diff.

Tom should have everything you need, glad you don't have a 63-66 housing since no vendor offers the correct case shims and never tell their customers that. Also get the correct full compliment axle bearings BH2212 no other.

I'll be watching.
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Old 11-20-2016, 06:52 PM   #14
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Old 11-23-2016, 07:58 PM   #15
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Thumbs down!

OK, everything that comes apart is apart


Center shaft has some wear (~.001" on the ends and .0020 - .0025 where the side yokes rub. I'll add one to the order list


Snow flakes were actually still in one piece but will get new ones anyway




Posi gears seem to be OK, at least don't have any broken bits or excessive wear that I can see/feel. I don't believe I've read much in the various threads about them except for those with actual broken off chunks so I just looked them over for excessive marks, gouges, ding-dents-damage and felt for flat spots or worn areas on the faces.




Cleaned the posi housing and went over it inside and out looking for any signs of damage or cracks, looks good so far








Outer faces of the housing seem good where the yokes would have contacted, if anything touched outside it wasn't much.


Once the grease and dirt was removed and a light brass brushing, this one side looks like it might have been kissed a little. Strange part is the area where it's not clean looks more like the casting didn't clean up during machining rather that corroded away. (or it was pretty selective rusting )


Anyway, not to let everything soak for a bit in greaser and then I can start deburring / polishing.

Order List so far for Toms

3.55 gears U10V-3.55
"installation kit" C10V-IK
(but I think the custom crush sleeve would work out better, I wonder if they'd substitute)
Cross Pin TCS-10V
18 Solid Disc and Plate Set C12-PP 18S
Posi Shims set C12-PSS
Thrust washer 10V-TW
(I think that's for the cross shaft retainer bolt)
Drain plug 1/4"-18NPT MP1

Still need to source a set of buttoned yokes and some ARP ring gear bolts.

Enough for today
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