My 1968 M715 build

[Troop ISP]

I purchased a 1968 M715 from a member a few months ago. He had the truck for several years and used it to off road. The previous owner installed a Chevy 350, new non-power master cylinder, and power steering. The truck was equipped with fairly new 38 inch Mudders, a Detroit Locker in the rear, locking front hubs, and there was a new (in the box) locker for the front end.

After reading quite a bit, I have decided to start my build. I have promised my wife I would not spend money unless I earned it working second/third jobs, or selling parts. Here is where I am at thus far.

Purchased original truck for $2300.
Sold stock axles and locker for $600.
Purchased 1985 M1008 for $900 (non running) and pulled the axles.
Sold the rest of truck for $300 (broke even).
Traded 38 inch Mudders for 39 X 15.5 inch Mickey Thompson Baja Belted tires.

I am swapping the 14 bolt to rear disc brakes. New spring perches will be welded and stock rear springs with a couple of leafs removed will be used. The front springs will be moved under the frame and Chevy 2 inch lift leaf springs will be used in front.

The existing engine sits very close to the firewall (the previous owner hammered in the firewall to clear the valve covers). I have a 700R4 that will be rebuilt to include a Corvette servo. The engine will be moved forward several inches.

I have been reading the November 2009 JP Magazine article on how to get 400 HP - 400 Lb/Ft of torque out of a Chevy 350 crate engine without any machine shop work. I am considering incorporating some of the suggestions into my small block.

I am currently cleaning up the axles for a fresh coat of paint, the onto welding in spring mounts in the front. The transmission will be next and then.....

Thanks for all of the great builds everyone is doing. I have gotten a lot of good ideas. The goal is to keep this project on a very reasonable budget. I have said that before though so we will see. I will try to download some pictures if I can figure it out.

[Troop ISP]

I will try to attach some photos. Don't know if it will work.

[IMG]http://i906.photobucket.com/albums/ac268/Troop2865
[IMG]http://i906.photobucket.com/albums/ac268/Troop2865
[IMG]http://i906.photobucket.com/albums/ac268/Troop2865
[IMG]http://i906.photobucket.com/albums/ac268/Troop2865

[snakeater]

what is the fascination with swapping out the stock axles? i see everyone do it and can't figure it out. i understand the more common 8 lug pattern, and disc brakes for the rear but barring those what are the real advantages? Gearing? Is the stock dana 70 that weak? Aren't there several m-715's with cummins engines running stock axles and 38 plus size tires with no problems? Educate me on this in real time.

[randyscycle]

what is the fascination with swapping out the stock axles? i see everyone do it and can't figure it out. i understand the more common 8 lug pattern, and disc brakes for the rear but barring those what are the real advantages? Gearing? Is the stock dana 70 that weak? Aren't there several m-715's with cummins engines running stock axles and 38 plus size tires with no problems? Educate me on this in real time.

The stock Dana front 60 is about the same strength as a Dana 44. Lots of twisted off front axles.

Your gear options are limited to 5.87, or 4.88 currently as I think that is all that is left.

No other wheels fit unless you buy Marsh or custom make them.

The drum brakes are OK until you really want to drive the truck more than its original 45 MPH. Only one manufacturer of discs and thats it.

For locking hubs, there is only one option left in Mile Marker and not cheap.

The rear 70 has some odd thin ring gear and locker options are limited as well.

Little axle tubes of about 2.5 inches in OD, vs 3-3-1/4" for stock GM 1 ton stuff.

If anyone does a Cummins swap and doesn't swap axles, they either aren't driving on the highway, or they are chancing blowing that 6BT to pieces at that kind of RPM. Plus the added couple hundred pounds up front can't help the weak axle.

[Troop ISP]

I could not have said it better myself. My main motivation for the swap was the following:

Strength, disc brakes, more reasonable gearing, aftermarket support, and common wheel bolt pattern. I definitely want to run larger wheels and tires. Other than Marsh or Stockton wheel, there was no other choice. With the Chevy bolt pattern, the choices are almost endless. The swap is fairly straight forward, and the axles are fairly inexpensive if you are willing to purchase an M1008 from Government Liquidators.

[Ronner]

If you are out wheeling and break anything on the stock axles you are basically screwed. With the swap to chebby axles you can get parts anywhere.

[MarknessMonster]

...more reasonable gearing... ...I definitely want to run larger wheels and tires.

Wouldn't the 5.89 stock ratio be advantageous to the folks that run tires taller than 38" or so -I'm not sure of the mathematical equation, but 39" might take the true ratio down to about 4.10? I run the 39" MT's with the stock axles/ratio, and I think that the gearing could definitely be lower (numerically higher). However, I also run an overdrive tranny, so I suppose that it would depend on how the truck was used if one didn't have an overdrive tranny. In other words, I only use 5th gear when I'm driving on a 65mph+ road, but a lot of these trucks aren't used for this type of travel.

It sounds like a nice build with a good resale potential for a modified truck, as the guts will be updated to more modern and replacement-friendly parts.

[Mikel]

Wouldn't the 5.89 stock ratio be advantageous to the folks that run tires taller than 38" or so -I'm not sure of the mathematical equation, but 39" might take the true ratio down to about 4.10? I run the 39" MT's with the stock axles/ratio, and I think that the gearing could definitely be lower (numerically higher). However, I also run an overdrive tranny, so I suppose that it would depend on how the truck was used if one didn't have an overdrive tranny. In other words, I only use 5th gear when I'm driving on a 65mph+ road, but a lot of these trucks aren't used for this type of travel.

It sounds like a nice build with a good resale potential for a modified truck, as the guts will be updated to more modern and replacement-friendly parts.

Going from the stock (36ish) tires to 39 would yield a 7.5% drop in RPMs, vs a 31% by going from 5.88's to 4.10's.

http://www.4lo.com has a pretty nice gearing calculator.

[Ronner]

5.87:1
With the taller tires you are right that they bring down the percieved ratio, and are even advantageous. The problem comes with those larger heavy tires though. The front D60 on our trucks is only about as strong as a modern D44. Would you want to run 44" mudders on a D44 with a heavy truck like this? I think Doug runs 36" tires and he likes to snap the shafts enough that I think he is the axle changing Zone champion! Or at least the fastest to do it in pink slippers
The CUCV axles are good because they are usually cheap, come with a Detroit locker in the rear already, and have the larger carrier so that if you want to swap R&P to something in the 500's you can.

[Ronner]

If I wanted a Micheline military tired everyday driver I might do something like a Ford 300ci straight 6 with a 5spd OD trans and a NP205 with the stock axles (a la Luckypabst).
That combo would net 2500rpm at 65mph while a stock truck switched to 4.10:1 R&P would net 2300rpm - but with the loss of the low speed pulling power from teh reduced gear ratio.