3/4 ton axles in the green machine

[Kiss My SS]

Perhaps this question has been asked before but I recently found a mid '70s 3/4 ton 4x4 and I was wondering what it would take to swap the axles into my 72 jimmy. I mean, Is this possible? Is there a conversion kit? How about the springs? Any help here would be great. It is getting hard to find a 68-72 3/4 ton 4x4 to take them from so I was hoping that newer axles would work. I would eventually like to put some bigger tires under the jimmy and I still want to be able to tow with it so the lower gears in the 3/4 ton should help a lot. Also, my friends all drive 3/4 tons so it would be nice to be able to swap spares on the trail. Thanks.

[JIMs70GMC]

First thing, the axles will bolt to the springs no modifications, no specail brackets nothing, unbolt your u-bolts slide new axle under and bolt back up. Next, shock mounts should be the same place front and possibly rear so bolt on your new shocks. Next brakes, your 72 has disc the donor has disc, the caliber from the half ton should fit on the backing plate for the 3/4 ton or you can upgrade to a 3/4 ton caliper, I think it has a slightly larger bore. Drag link is the same so bolt it on to the steering arm. You will need a hybrid u joint that will allow you to connect the 1/2 ton driveshaft to the 3/4 ton yoke of the front diff. Same for the rear. In the rear also, the center section of the 3rd member could be longer and could require a slightly shorter drive shaft, Sounds like you probably have a 12 bolt rear going to a 14 bolt rear, correct? One spot that could be a problem will be the rear shock mounts on the axle might have to be moved. All I can think of, anybody else?

[Beast388]

Jim covered it pretty well except for one thing: The front axle from the mid-70's 3/4 ton 4X4 with be a direct bolt in. The rear axle (14 bolt full-floater) however, will require a bit of modification. The leaf spring perches on the 14BFF out of a 3/4 have a center to center distance of 42.5" Our trucks require a spacing of 40". So you will need to cut off the old perches and have new ones welded on at the correct spacing. Also, as Jim mentioned, you'll need to move the shock mounts as well.

I have been researching this very subject for a while, so if you have any questions, just let me know.

I will be doing my axle swap in the spring.

[Kiss My SS]

So if what I understand is correct, the front springs, axle and steering will all bolt up. All I need to do is take out the old one and put in the new one and get a modified front drive shaft. As for the rear, the 3/4 ton springs are too long but the shocks should bolt up if I put the 3/4 ton axle on the 1/2 ton springs it will work. I am not worried about brakes, I would use the 3/4 ton ones, not worry about switching calipers. It is easy to connect the lines and bleed the brakes. I was wondering about this. Use the 3/4 ton front springs, axle and steering from the mid 70's truck and the rear 14bff but then get new 3/4 ton rear springs for a 68-72 truck. That way I have the heavy duty suspension and axles that are all bolt ons. If I ever wanted to swap back to the 1/2 ton stuff then I could. Lemme know what you think.

[HeavyK20]

I'm not sure on this but I think the front U-Joints are the small ones on both 1/2 and 3/4 ton. If that's the case you can use either front drive shaft, assuming bath trucks have the same transmissin/transfer casle setup. As for the rear, if both transfer cases are 205's you should be able to use the big yoke from the 3/4 ton and then just have the 3/4 ton drive shaft shortened.

Good Luck

[HeavyK20]

Actually the rear springs are not too long, they are too wide. If you take a measuring stick and measure from the center of the driver side leaf to the center of the passenger side leaf you will see the difference. So the spring perches that are welded to the axle housing will have to be moved to match the 72 Spacing. The shock mount that is on the rear of the mid 70's will need to be cut off and moved to the front of the axle housing. Not sure about shock spacing between the two, but I would guess is does not have to be exact as long as you can mount the shocks.

[Mike C]

It is the spring spacing that is too wide and not the springs...Note that the 2 wheel drive leaf 67-72 trucks have 2 1/4" wide springs vs the 2 1/2" wide springs of the 4x4. 73 and up one ton trucks have 40" spacing on the rear axle. Not sure about other 4x4, but you will have to move shock mounts, but that is pretty easy.