12 bolt power limit questions

[ibinyth]

I have a 12 bolt in my 70 c10, just rebuilt it with trutrac 3.73, moser 30spline axles, and a bearing preload screw cover. My question is what sort of power can I expect this thing to handle and does an automatic make a difference vs a manual?

[vince1]

I know on new trucks autos have higher rated pulling capacities than manuals. Smoother power to the wheels?

Maybe your differential will be OK with spinning the wheels a bit. Maybe not so good trying that if you have a lot of weight on it. Are you planning on pulling with it?

[Ziegelsteinfaust]

The weakness in the truck 12 bolt was the pinion diameter. Or its biggest difference from the car version.

If your running street tires under 30" diameter your good for 500-600 hp. Alot of that depends on how sticky the tires your using, and how hard you launch. So there is no exact science on this.

The bearing support cover is among the best things you could of done. When the caps walk it tears up the weaker pinion which is what typically dies under repeated abuse. Personally I have never had a axle break on me, but I tore up several gear sets. A few were likely from cap walk. Opening up pinion/gear seating, and well the rest is history.

Other things that change durabilty big time. Are heavier, and taller tires. Heavy or overloaded vehicles. Running a C10 when you should of gotten a C30.

As of late I have become a fan of the brand F 8.8 rear end. It has large spline counts, and with weld on 9" ends. To me it is the best light duty rear end to build due to economic concerns You can use a bearing support cap, and studs. Only down side is like GM rears no easy gear changes in a couple hours like the 9 inch. Sure you can swap the whole axle, but it is a bit more involved.

[Ironangel]

The GM Truck 12-bolt is a VERY stout rear end. I've been pounding one with a 396 and a 427 for years and it's held up nicely. I welded the tubes, replaced the trailing arm perches with CaptainFab's perches, and added the Moser cover. They are VERY reliable. The day I break mine is the day a full floater Dana 60 or Corporate 14-bolt will go under the truck, just rubs me wrong to stick a Dorf anything under my Chevy...

[sweetk30]

you can upgrade the main cap bolts for a stud kit for a ford 8" if i recall . this holds them better than the stock bolts do . ebay for 25-30 bucks a set new .

yep found it in my build pics i saved . arp kit 250-3008 part # . and i did aftermarket HD 4x4 style spring pads for longer length to help control axle wrap . and less prone to crushing . also found the stock perches were off from each other over 3/4* in there clocking . i also did the detroit true trac and TA cover on mine .

have yet to run my setup as the truck is not done yet . but hope to at some point . . . .

[GASoline71]

Originally Posted by Ironangel

The GM Truck 12-bolt is a VERY stout rear end. I've been pounding one with a 396 and a 427 for years and it's held up nicely. I welded the tubes, replaced the trailing arm perches with CaptainFab's perches, and added the Moser cover. They are VERY reliable. The day I break mine is the day a full floater Dana 60 or Corporate 14-bolt will go under the truck, just rubs me wrong to stick a Dorf anything under my Chevy...

This ^^^

These are great rear ends that will take a lot of abuse. People really tend to sell them short for some reason. I have yet to break one, and I am not easy on mine at all! I added the Moser diff cover as well.

Gary

[ibinyth]

I appreciate all the input fellas, I'm not too terribly worried about it. Just curious how much abuse it'll be able to take, it's mostly a street truck. That being said I know how I drive and the next motor will be making a minimum of 500 wheel and up to 650, I'm going to attempt to keep it at the 500 mark but no promises. The truck doesn't haul anything, bedsides are tube mounted and no bed floor, unfortunately my suspension setup in the rear mimics pchrods c10r so the axle perches are there to stay. I'll see if I can figure out a way around it to prevent axle wrap. Tires are a 315/30r18 on a 18x11, truck isn't together yet just now getting the time to start putting it back back together. It's a ways away from the road, but info like this helps in making sure it lasts a while.

[sweetk30]

https://www.calvertracing.com/caltracs.html

heard good stuff about these guys . if i have problems its my 1st choice to go to .

[burnin oil]

Transmission towing capacity is mostly due to maxing out the transmissions design strength. Dodge was the last manual 1 ton and they limited torque to around 600 foot pounds. Demand was low for a high strength manual so autos got redesigned to take the 1000 foot pounds that the motors are making now. They also use torque management tuning to lessen the bang at shifts on an auto but the manufacturers had no control off the average idiot dumping the clutch. A heavier duty manual could be made but shift quality would be less so it was cheaper and easier for the big 3 to dump them.

As for the 12 bolt strength. I broke one basically. I tore every bearing out of it but the gears held. You know that you are hooking it when you drive the axleshaft through a brand new Timken axle bearing repeatedly. I got to the point of changing them on the side of the road when I was in high school. For all the abuse that truck took I don't think you will have an issue until you start to run cheater slicks.

[Ironangel]

Matching the gear set to the motor and transmission is key to longevity of all three...The more HP that motor spins up, the lower the gear you can run. In other words, you don't necessarily have to tear the guts out rear ends and transmissions with 3:08 gears when a 3:73 or 4:11 would have lived transferring HP to the tires. I'm running a 4:56 with a built Turbo 350, and yes I have a 400 THM for when the 350 lets go. This new motor will undoubtedly find the weaknesses...But the transmission and rear end have lived through two motors pursuing my 12 second dream...

[vince1]

When I got my truck I had to tow it home across the road backwards with the pinion flopping around. Towing forward the wheels would slide. I guess they ran it out of oil so the 250 had no trouble trashing it. It was a 3.73.

The replacement 3.07 was full of some substance that felt like sand. I'm getting more life out of it by having cleaned it out and changing carrier bearings and axle bearings set to run in a different spot. I'll have to see how it lasts with 2 more ponies in front of it.

I did see a Ford diff (not a 9") break more than fifty years ago when some kids were giving it all they could while driving it around town. The carrier cap bolts broke on that one so maybe studs would have been better.

[Tom]

Anything can break. I blew up a Sm465 transmission that everyone thinks is bulletproof. That being said, the stock 12 bolt in the truck survived and is still going even though the trans died.

[burnin oil]

Originally Posted by Tom

Anything can break. I blew up a Sm465 transmission

That takes skill! The only one I have ever seen broken internally had the syncros moded by cutting off every other tooth for speed shifting. Not mine. On my 465 I did have to install new sintered bushings. That was In the same truck as the 12 bolt that I tore up. Considering I torqued it enough to bend the floor pan on more than one occasion I am surprised that I never broke the mount bolts. Always driveshafts, axle bearings, or ujoints.