Cracked open the diff on my BB restore project.


Good news is nothing is broken or cracks evident. 336 gears look OK. 90k orig miles with mild BB. 72 orig posi. Looks like bigger spider gears?
Now for the big question:
How much should I beef it up for a strong BB at 550ftlb?
Tremec CR 5 spd, street tires only. Maybe 1 run at the drags just for fun. No more autocrossing.
I am thinking about cap screws, ARP ring gear bolts, tuned posi, polished housing (no sharp edges), solid friction plates, and maybe a solid spacer.
Side yokes have .025 & .029" clearance. Would rather have tighter.
Do I need new ones?

 

If you go to Toms build videos on Youtube you can gain a little bit of info on your questioons. He likes to build the rears to .005 tolerance on the side yokes but there isnt a build spec, just a replace spec. Im sure some of the racers will chime in momentarily. The good news is you arent goiing to blow anything up using street tires, you just wont get the grip that would cause the diff to eat itself.

 

^^^^ If anything regular street tires are your fuse...spinning may keep anything from breaking.
Keep us posted

 

Say Leigh, I like your "vertical" chassis rack - it looks VERY familiar to me. How's it working out for you? I hope you like it!
If you throw a piece of plywood on the dollies in the interior of the rack you get extra storage space for parts/supplies...
Happy Holidays,
Paul

 

Search the forum for posts from GTR1999. Gary knows more about these than just about all of us combined. He was incredibly generous with his advice when I tackled mine last fall. I built a period correct L-88 that dyno’ed at 505 HP/500 lb-ft and upgraded my diff following Gary’s posts and his direct communications with me.

I seriously considered building a Super 10 but ultimately decided that for my intended use of the car (not going to be raced or abused), it wasn’t worth the cost, plus the fact the 30 spline spiders Tom used to sell are no longer available (unless you have Gary build you one).

I ended up putting in a 73 posi case (the one on the right before I “de-stress riser’d” it), new Tom’s 17 spiders, new 3.73 US Gear R&P, new Timkens, ARP ring gear bolts, Tom’s steel cap (LH only), Ratech solid spacer. I tuned it as Tom’s video shows, no preload plates/springs, Muskegon cover. I was fortunate that my old HD axles are pristine so have 0.005” gap to the cross shaft. My posi plates all measured out at at 0.068 +/- so reused them. I set the pattern just as you can see in Gary’s avatar.

I put on a 1330 pinion yoke and am in the process of ordering a Denny’s Stage 2 driveshaft with a Mark Williams 1350 detachable front yoke (mated to a TKO-600). All non-greaseable Spicer U-joints.

I spent a lot of time thinking about this and am very confident I have a driveline that I don’t need to worry about when I press the go fast pedal. That said, I am also respectful of the fact that it isn’t a Super 10 (or 12 bolt for that matter), nor do I have the 30 spline costalotium axles and expensalloy half shafts.


69 on the left, 73 on right

 

A lot depends on what you plan to do with the car. I see the mods you list are what I use so you're on the right track but lets look at some options.

What you have now, a stock 72 diff. It's a BB diff and that only means it originally was stamped with a F code and has "HD" axles. The HD axles are just cap and bolt axles, they are good but sure are not a deal maker for a HD diff.

550 ft/lb is enough to break a lot of parts if you abuse the car. I have rebuilt plenty of stock that were wrecked with street tires as well. Shock loading abuse with an engine over stock rating will find the weak link in a drivetrain designed around 300-400 hp 60 years ago.

So you can buy a master kit and posi kit and do what I call a vendor rebuild. They are pretty basic builds done fast to meet a profit line. If you don't abuse the car they will work. I pesonally wouldn't build a vette diff without polishing and radiusing it, or correctly tuning it including clutch run in - but many use these builds without a problem. If you do all the work you can get it done for under $600 in most cases. Just understand what you have.

You mention polishing and tuning, be sure to watch Tom's old video on it. Tom has been gone almost 3 years now. some think a window with a little radius is polishing, it's not. Some think leaving out the spring pack is tuning- it is far from it and if you don't get them balanced correctly will not work. Polishing and tuning takes me a good 3 - 5 hours sometimes more. Every posi case is machined differently so sometimes a stock set of posi shims works other times they need to be ground to size. To do this correctly you have to have access to a surface grinder or get lucky where the 005 increment shims will work. Sometimes they are within 001-002 of each side others they can be out 020"

Axle end play - I set mine for 005-007 but that depends on several things. I have seen some rebuilt diff's with 050 endplay. I have typed out this info so many times I really don't feel like doing it again but you can find it easy enough. Your axles may be good or may need to be hardened but I didn't see them rolled back in your picture. An original, hardened GM axle from 63- about 70 will be about the best 17 spline axle you can use today. If yours were worn a lot more then rebuilt axles will work, new International axles are very good as well.

The spiders are not larger, they are 10-17 tooth count which started in 72. They are larger then the junk found in the vette DANA posi's but the same size as the 10-18 used from 65-71. 18's will work in your application level but the 17's as you have are better.

A solid sleeve replaces a stock crush sleeve and is used for drag racing with hard launches. If you need a solid sleeve for actual use then you need a steel cap on the LH side, they go hand in hand for that type of application and both need to be machine fit with a complete understanding of what they are supposed to do. A steel cap for the RH side is not required - no matter who says it is. It is a con -pure and simple. It won't hurt anything but it isn't going to do anything extra for the RH side other then look pretty, some like that and pay for it.

So again it depends on your intention. Just cruising and going through the gears with BB is one thing, really loading the car, will find that weak link every time- snubber brackets, sheared outer axles at the threads, imploded posi cases, broken u-joints, and cracked frames are some things that I have seen a lot of. Sometimes upgrades to a real super 10 or 12B are required along with the other reinforcements but it doesn't sound like it for your use.

You're on the right track though, doing your homework before you start spending your cash.

Good luck to you.

 

L88 - turn your posi cases around if you have pictures, then you can see the difference in the cases. Inside the 69 still retained the dual seam casting while the later cases did not. None are crack proof but the later cases are less likely to crack. Given the option of the two, I would take the 73 every time. Also mill the pad on the case a little deeper for better support. For added insurance cryo treat them. I see REM polishing mentioned from time to time too. REM polishing is used for Aerospace parts and smooths them nice but really not needed in a street cae. Again, won't hurt but for anything other then a real road race car all it does is make the owner feel better. I have nothing against REM, they are a quality company here in CT and the automotive teams use it a lot.

 

Thanks for the tips.

I am sure I will be talking to you as I "dive in". I have never done this before but I am all about educating myself about the options.
I do want to do at least 90% of this myself, that is the whole point for me, the learning and the doing.
If I don't need the solid spacer or the billet cap then I won't do it. The widest stickiest tire I will ever put on it would be a 275 BFG G-Force or similar. Never slicks. I'll be starting with BFG TAs on stock Rallys and see if I have "any" traction LOL. And upgrade tires/wheels if it is just too "slippery".

But if anyone thinks the solid spacer, or the billet cap, or bigger halfshafts or anything else makes sense I'll do it. After all I will have a 5 speed with a 10:1 overall first gear. My buddy built a real torquey motor for me, and I would like to be able to "lean on it" without scattering parts. If it will probably not break, I would like to run it down the qtr once or twice, just because. But I have never been a drag racer and do not launch it hard. So with that said it sounds like the billet cap and solid spacer might not be required? I don't "shift hard" either. I powershifted my LT1 occasionally but do not think this drivetrain would handle that abuse.

I will do all the grinding/polishing etc. But I will have my machine shop buddy set the pattern or install the caps/cap screws. I saw a couple of your split-in-half posi case pictures - a great education!

It "sounds like" your larger diameter socket head cap screws would be a good addition?

Not too interested in pulling the rear 2 or 3x to get it right. Once and done for me!

 

So is there a way to examine the stub axles to see if they have too much wear? See pics. The toothed section above the c-clip slot appears to be .188-.190" tall.


Here's a close up pic of my stub, and a new Eclkers "Hardened" one. My tooth "slant" appears shorter.

 

If your friend has a Wilson Rockwell hardness tester check the hardness of the axle faces. 50 Rc is good. If the dimension is 190" then there's little wear on them, they don't look bad. If they are soft but in the 190 range you can source a heat treater and have them hardened and tempered.

You can use 7/16-14 socket head cap screws in the bearing caps, it's cheap upgrade. Steel caps use 1/2-13's. A solid sleeve and steel cap are for hard launches, if you are not planning on that you don't need them. You are less likely to break things if you are moving vs a sudden hard shock load. A few passes at the track should be ok but watch for bouncing in the spring, enough will cause issues. When you launch, the rear squats and load moves inward against the cross shaft. IRS should be rigid but that comes at the expense of driving comfort too. Adjustable shocks will help for different use.

You can certainly polish a posi at home just don't do it like some who claim it is polished and only spend a short time on the window edges. If you do it do it right. You will need a good set of grinders, wheels, and a mini belt sander works good. Polish the journals and flange in your buddies lathe. Countersink the flange holes for ARP bolts. Mill the posi pad deeper

You can run GM 1/2 shafts in either size for you application. There is a widespread picture of a twisted 1/2 shaft online, look and you will see other mods as well so the application has a more power and TQ then yours. Typical shaft failure is with the u-joint not the shaft. We broke a Tom's shaft, it was not the shaft but the solid spicer joint sheared under a hard launch, taking out the shaft yoke- the real weak link area more so then the tubes.

Hard use will snap the outer axles at the base of the threads. Some imported axles broke there with little abuse. I know of one guy who had it happen just driving on the street, it wasn't from abuse it was from cheap imported parts of low quality.

 

Thanks Gary!
Just what I was looking for. Good effective common sense upgrades, without getting carried away.
The only thing I did not understand was "Mill the posi pad deeper" ?

 

remove the cross shaft bolt. lay it outside the case where it would be in place and look to see how far through the cross shaft the bolt extends into the case. The Eaton cases are shallow, just about everyone out there, sometimes they break out the case and break the bolt. Milling the pad lowers the bolt and extends it into the case. Something I do on every posi I build.

 

I had the cross shaft retention bolt pad milled down 0.120”. When I trial fitted the retention bolt after, it bottomed out in the case so I ended up trimming the bolt a couple thousandths off the tip. Be sure to check that if you choose to make this modification.

I chose to use the solid sleeve not because of strength concerns but because I found it easier to work with when setting up pinion preload and adjusting pinion depth, avoiding the hassle involved with using a crush sleeve. An added benefit is if you ever need to change the pinion seal, you know that you’ll have the same pinion preload after you torque it down.

 

Once the pad is milled the hole needs to be drilled and tapped to allow the bolt to fully seat. You want to get full extension into the case.

I went back and pulled some pictures, since Sunday Night Football is over now.
Here is a bolt on the case you can see there is little extension into the case.



After milling, drilling, tapping



Case broken out

 

Great I understand how tuning the posi and moving the spiders toward the center would decrease the end-play. So maybe these stub axles will be OK after that. Now I just need to figure out if they are the later "soft" ones....

 

Where can these videos be found? If they're on Youtube, what's the name of the channel?

Thanks.

 

I found the videos just by searching for "How to tune a posi"
There is part one and part two:

 

In another post I found a description of a Chinese axle vs a Spencer Forge (IIRC)
I just had a shop put a new axle in one of my trailing arms and does this fit the description of a chinese one?
And Should I worry about it for my 500 HP street car?

 

So to follow-up.

I found out this imported axle was sourced by my vette shop (Ron at Corvette Paramedics) from Van Steel. We called Van Steel together and after a nice talk, Paul at Van Steel assured us that they have been using this one particular supplier of axles for several decades. They are actually from South Korea. They have had them heat treated and rockwell hardness tested and their is barely any difference at all between them and OEM GM ones. Certainly less than 5%. In his opinion (40 years of working with vettes) there is no need to pull them to go worth the USA made ones. Almost no gain. (Although they both would be glad to sell me the USA ones if I insisted.) . In short, He would run them in his car, if it this was his car..

Case closed.
And Kudos to both men at both companies! Two outstanding suppliers and we are lucky to have them!

 

Depends on use. Many GM axles have sheared at the base of the threads with some abuse. I know of a guy whose new imported axle sheared a few months ago without any abuse. 500hp is about the point where they break but again it depends on power, trans, traction and your right foot. Nothing against either vendor , as long as you're happy good to go. I have never had a Toms 31 break yet. Rockwell testing is just that, a test of the hardness of metal but that meaningless unless you know what level it is on the C scale.