12 Bolt Differential build

[GTR1999]

Hi Guys,
I detailed this on another site but figured some here may want to take a look.

http://www.vettemod.com/forum/showthread.php?t=2090

 

[Ed's 1970]

Thanks for the link Gary. At what point do u need 31 spline spindles, when running slicks?

 

[GTR1999]

Hi Ed,
Well, there has been some debate at what point the 10 bolt and IRS start to fail. This of course is subjective since power alone will not break things, driving style will. I've seen posi's cracked and U-joints broken with 300-375 hp and a fair amount of abuse. When asked for a "transition" point I say about 500 hp/TQ. At which point thing may start twisting and moving under load. Again this can't be written in stone, but I feel it's a good range.
This job is for a 650+ HP car and will have the 12 bolt,3.5 1/2 shafts,31 spline spindles. Pretty strong parts that should work out great.
I have posted about this job on http://www.digitalcorvettes.com/forums/showthread.php?t=110048 as well. I could post pictures here but the links save me time.
I really like these parts too, I may have to build one for one of my vettes! There is a fair amount of fitting and machine work with them too.

 

[Hib Halverson]

While 12-bolt conversions have been around for the 63-79 diff for many, many years and I agree that, at 650 hp and espeically with any kind of decent tire under the car you'l l need it--if this is for a drag racing application which will get raced at an actual drag strip, you'll want to check the track's sanctioning body's rules. Some racing organizations ban IRS on cars which run under a specific "index". I don't know what those indexes are but my guess is with, say an NHRA track, there will be an e.t. that, if the car runs quicker, you'll be required to convert the car to a straight axle in the rear.

On the other hand, if were you're going to race has no such requirement then the 12-bolt conversion is a must as is beefier half-shafts, stronger axles and bigger wheel studs.

 

[GTR1999]

Hib is correct, 12 bolt setups go back to Pepe Estrada in So Cal in the late 60's, then Tom Watt bought him out and has been building them the past 35 or so years. Tom is no longer building them so I have started. Tom has them in 7-8 sec cars. Hib is also correct to check into the requirements at a track. Most of the guys I build stronger diff's for are using them in street cars though and want the strongest IRS diff they can get without going to a solid setup. Once you have both stock and the HD parts next to each other they speak for themself. Real nice stuff that Tom put his lifetime into revising and building.

 

[Hib Halverson]

I know Tom basically retired from axle service work and moved out of the L.A. area a number of years ago. Tom did the diff in my 71...twice in fact. His work was excellent back when those two rounds of mods were done, but in the last couple of years he still did service work, I think his quality slipped manybe a little...that's based on a lot of problems I had with a conventional 12-bolt axle he did for my Chevelle with an engine of only 425hp. I think Tom wold have been better doing what he did (retiring from the service work and just selling parts) sooner.

Now, are you just doing 12-bolt conversions of the 63-79 10-blt axles or are you also doing work on diffs with the OE gears?

Right now, my engine makes about 530 lbs ft and I'm running the 10bolt with a 3.08 gear and a few of Tom's mods including his tuned-up limited slip. The trans is an ROD six-speed. I'll probably keep that diff even after I move the engine to about 575-600 lbs ft because I'm not putting slicks on the car. The most aggressive tire I'll run is a drag radial and probably won't be sidestepping the clutch, either.

If I break the axle...well then...maybe I'll need a 12-bolt conversion.

 

[GTR1999]

Hib,
I do custom 10 bolts and now the 12 bolts for the 63-79's. I polish and tune all the posi's I work on along with the other custom parts to beef up a 10 bolt. I can machine the 10 bolt posi case for a 12 bot cross shaft and spiders as well.

The custom 10 bolts should be good for cars with less then 500 HP and st tires. Once you get to 500+ and the tires start to bite in then the stock setups are going to break. The 12 bolt,as you know, is stronger but anything can be broken. I never like to use that term "bullitproof" as I'm sure you're seen some strong parts broken over the years on many cars.

As for Tom I couldn't answer for him. I know the parts I use are stronger then stock and I think are the best on the market. Most are Tom's parts in fact. I know the reasons why he's no longer building but that's not for me to say but service quality isn't one of them.

 

[Ed's 1970]

Gary, thanks for all the info! I plan on completely redoing the rear-end components and trailing arms, as I am currently doing a body-off resto. What parts would u recommend to beef up the stock 10-bolt rear without spending a fortune, that 12-bolt looks like too much work. How about the spindles, are the aftermarket ones with the 17 splines strong enough? I believe the torque rating of an axle goes up with the more splines, so I was kind of thinking about going with 30 spline (inner) axles and retaining the stock-type 17 spline spindles. I already have 3.0 inch halfshafts, with a .134 wall. I would say at this point I will not be able to afford to race this car, so street use only with street tires. I am putting in a mild 540 big block, so it is going to have a lot of torque.

 

[GTR1999]

Ed,
A lot will depend on how you drive the car. Once you get to 500+ then things tend to break, of course this will vary with car to car.
I never build a stock 10 bolt anymore, I polish and tune all of them now. There are plenty of places to get a stock built unit but few take the time to beef them up. If you stay with a 10 bolt, I would tune,polish,use ARP bolts, Toms gears, machine the 10 bolt posi and use the 12 bolt shaft and gears, maybe add the steel cap for strength. The 30 spine axles are better but most use the 17. The 31 spine spindles are much better, look at the diameter compared to stock 17 splines. Plenty use stock spindle though for st/strip cars just be aware of what you have.

 

[GTR1999]

If anyone is interested in this, I just finished it up so check the link for more pictures. It came out pretty nice and I added a couple of my own mods to it as well.