cummins with a 12-bolt

[79GMC_OLDS]

ok heres the deal...i have a 79 gmc 1/2 ton 2wd and im putting in a 5.9 cummins and an allison at540 from a school bus. With this combo there should be 600-650 ftlbs at the wheels. The allison is a non od 4 speed.

So with no od , 28" tires and a max rpm of 2700 i need to run a high rear gear for driveability. I have a 2.73 open 12-bolt truck rear end to use. So question is... what do i ned to make the 12-bolt hold up? I cant run a dana 60, 70 or a 14 bolt gm because of gear choices. I was thinking posi so at least the torque load is split between the 2 axles. Does anyone makea 2 series posi? Will the axles and gears themselves hold up or should they be upgraded as well?
thanks

[grinch4om]

I wouldn't use the 12 bolt, ain't that strong, not like the old camaro 12 bolts, I'd do a 9" out of a ford pickup, the wheel bolt pattern is 5x5.5 but cheap to drill to the chevy 5x5 parrern . There are tons of stuff for those rears and they will take the poop that cummings puts out

[Zoomad75]

All you can upgrade on the 12 bolt is moving to chromo axle shafts. Problem is the 12 bolt is still a semi-floating c-clip axle. Meaning if you did break an axle, the whole axle shaft is coming out with the wheel attached to it. With that much torque it won't take much to snap stock shafts. Add a locker too it and you will be breaking twice as much stuff. At least open your next weakest link behind the axle would be traction. As long as you spin and not hook up it won't be as likely to break. I know that statement sounds backwards, but GM's been known to make little axles live with big power by letting the tires be the slip point to the ground. (4th gen camaros with 350 hp and 7.5" c-clip 10 bolts are a nice example) Warn did make a full floating axle kit for truck 12b axles, but they are discontinued and really hard to come by now.

If you must have the highway type gears you might want to look at just a shade lower gearing. 3.21 ratio is available for the 14b full floater which can live behind the kind of torque that cummins is going to throw at it. Its not 2.73's but you could run a slightly taller tire to make up for the gear ratio. It would require changing over to 8lug wheels out back and up front if you wanted them to match.

One other option that gets overlooked for the full floating 14b is it's 14b semi-floating cousin. It's got a 1 inch smaller ring gear than the Full floater and it's still a c-clip axle but some came as 6 lugs in the 1988-99 vintage trucks/utilities. Mid to late 80's 3/4 ton suburbans had them too, but they were 8 lug. Highest gear ratio listed is 3.23. find one and get some axle shafts redrilled for 5 on 5.

As stated above, a custom built 9" ford axle is a NON-GM option. Light and built to take a snotload of power, you can get the axleshafts drilled for the non-ford bolt pattern. It's still a 1/2 ton axle, but the axles bolt in and are not held in by c-clips.

Your asking a whole lot out of a 12bolt to throw 650 pound/feet of torque at it. It will break, just when is the only question.

[Pyrotechnic]

My truck for example, in 3rd gear with 3.08 gears and 28" tall tires, 2800RPM caculates out to 73.4 MPH. Not counting torque converter slip.

You should be able to reach highway speeds with 3.23 gears and taller tires but if you had to pass someone you really couldn't as you'd be topped out. Probably not going to be good on fuel running topped out all the time either.

You're either going to have to buy a 2.73 rear end that will handle that power or a transmission with overdrive. If you like shifting your own gears, theres the NV4500 5 speed that the Dodge's came with that should bolt right up and is plenty strong, and you even get a granny gear. Find out which option is cheaper for you.

[hiway]

I'm just courious about the 1/2 ton front.

[Zoomad75]

Originally Posted by hiway

I'm just courious about the 1/2 ton front.

Umm, it's not a problem with the power as he stated the truck is a 2wd. What the wieght of the big cummins is going to do to those front springs is a whole other problem!

[hiway]

That's what I was refering too.....

[Pyrotechnic]

Personally for a Cummins swap, I would start with a 3/4 ton or even a 1 ton dually as the truck should already be able to handle the power. I would also go for a 4x4, because if you have that much power and a heavy truck, why not be able to get the power to the ground when you really need it? 3/4 or 1 ton will have a suspension that will support the engine, a drivetrain that can handle it, and brakes that can stop it.

If you just want to run a diesel, GM 6.5 Turbo Diesel with a 4L80E would be a much easier swap. From what I understand, the mechanical injection versions are pretty reliable and the electronic ones can be too if the PMD is properly relocated and cooled. It would basically bolt in and a 12 bolt would be fine. It's not quite the medium duty industrial beast that the Cummins is, but with your truck the way it sits it would be right at home. The Cummins will need quite a bit of work to do the swap right in a 1/2 ton.

[boatpuller]

Originally Posted by hiway

That's what I was refering too.....

no doubt............i've had a dodge with a cummins,their not light.i drove it on the beach alot they don't like soft sand.loved the motor,not sure i liked the fact it came wrapped in a dodge.cummins stuff never failed me the dodge stuff did though.

[79GMC_OLDS]

ok thanks guys...all the work is done to put the cummins in the truck alls i need is my tranny and it will be a done deal. I am too far into this to be thinking about 6.5 (boring), an NV (dont want to shift gears) so i have narrowed down my options lol. Sounds like a 9" ford is where its at. I will do some more research.

[Pyrotechnic]

Originally Posted by 79GMC_OLDS

ok thanks guys...all the work is done to put the cummins in the truck alls i need is my tranny and it will be a done deal. I am too far into this to be thinking about 6.5 (boring), an NV (dont want to shift gears) so i have narrowed down my options lol. Sounds like a 9" ford is where its at. I will do some more research.

Post pics! Cummins swaps are always interesting!

[porpaintt]

You will be sorry for using that 1/2 ton. I looked at the possibility of dong that and went with a 3/4 ton the 14 bolt has 3 bearings on the pinion and will handle anything that cummins can throw at it. A dana has 2 pinion bearings and is about 1/2 the size of the 14 bolt. same with ford 9".
I don't know if I would use that allison either they wre plagued with problems and expensive to fix.

[79GMC_OLDS]

Originally Posted by Pyrotechnic

Post pics! Cummins swaps are always interesting!

ok ill try to get some for ya

[79GMC_OLDS]

Originally Posted by porpaintt

You will be sorry for using that 1/2 ton. I looked at the possibility of dong that and went with a 3/4 ton the 14 bolt has 3 bearings on the pinion and will handle anything that cummins can throw at it. A dana has 2 pinion bearings and is about 1/2 the size of the 14 bolt. same with ford 9".
I don't know if I would use that allison either they wre plagued with problems and expensive to fix.

I am well aware that a 14 bolt rear would hold up alot better. As i stated i am trying to avoid it because i can only go as high as a 3.20 something gear. I may have to settle with this though in the end.
I dont see why i would be sorry for using a 1/2 ton. I am only adding at most 300lbs to the truck from the engine and 50lbs from the transmission. With good tires and weight in the box it isnt hard to get the power to hook up with a 2wd truck. I realize that it wont handle like a sports car but i dont ever expect a 5000lb brick to do so. Springs/shocks will solve alot of the front end problems i may experience, and carring 350lbs of extra weight will not effect my stopping a terrible amount.
I do not know any history of the allison at540s as far as reliability but it worked fine in the school bus it originally came from. Its life in a vehicle less then half the weight shouldnt raise too many issues i hope.

[porpaintt]

That 5.9 weight is 1100 lbs. a small block weighs 550 lbs a big block weighs 725 lbs. the front end (control arms&A arms and spindles) are half the size of the 3/4 ton & 1 ton.
I hope it works out for you.

[hiway]

That's more of what I was thinking...HEAVY motor. Anyway, I've thrown some serous abuse at 1/2 ton rears and only broke one axle. Slicks, spray and all. I would try a 2:73 geared 12 bolt and see how it holds up. You never know. Still more worried about the front for ya than the rear.

[Pyrotechnic]

Is this truck going to be just a daily driver or will it be hauling/towing large loads ?

[SanitysBane]

I agree on the Ford 9. It doesn't take alot to make them handle large amounts of power. I did find a 2.80 gear for the Ford and a 3.08 for the 12 bolt.

[79GMC_OLDS]

it will just be a daily driver...the 350 diesel it is replacing weighs almost 800 i thought? 3/4 front end components bolt up anyway dont they? If thats my biggest problem then im not worried. I have decided to use the 12 bolt and then when funds allow put in a 14 bolt 3.21 with 30 or 31" tires in the rear. If 3/4 stuff works up front then ill also upgrade the front and run 8 bolt wheels all the way around.

[SanitysBane]

3/4ton stuff will bolt right in place.

[79GMC_OLDS]

awesome thanks...im still working on some pics. This project still has alot of work left in it but should be sweet when done.

[79GMC_OLDS]

ok well deal on the allison fell through
so good news is i have decided to run an nv4500 so i can se a good rear end.
Has anyone done this swap? My mane concern here is ho the nv clutch master cylinder was adapted to the stock clutch pedal.
Anyone have a set of pedals for a 73-87 truck?
thanks

[Russell]

You'll have to get an advance adapters bellhousing to move the clutch slave over to the driver's side of the engine, or drop in a Ford front diff to move the driveshaft to the opposite site, and get a driver's drop t-case.

Other than that, simple swap, just grab a set of pedals from an 85+ truck with an SM465, and you've got your hydro setup. Bolts straight into place of an auto truck's pedals, just need to drill a 2" hole in the firewall for the master cylinder to sit in, and the two holes on either side for bolting the master up.

[79GMC_OLDS]

Originally Posted by Russell

You'll have to get an advance adapters bellhousing to move the clutch slave over to the driver's side of the engine, or drop in a Ford front diff to move the driveshaft to the opposite site, and get a driver's drop t-case.

Other than that, simple swap, just grab a set of pedals from an 85+ truck with an SM465, and you've got your hydro setup. Bolts straight into place of an auto truck's pedals, just need to drill a 2" hole in the firewall for the master cylinder to sit in, and the two holes on either side for bolting the master up.

well i have a location on a dodge belhousing, clutch/flywheel and adaptor plate so ill just use that with the chev nv4500. The truck is 2wd so i dont have to worry about the other stuff.
Does the stock pedal have enough travel for the nv4500 clutch or will the pivot point have to be modified?
if it has enough travel do i just drill the hole in the firewall so it lines up with the mount on the clutch?
thanks

[blueriverred]

you could try a gear venders under driv over drive unit like i have in an old dully