j20 front axle swap w/ 1970 j4000 closed knuckle

[j400068]

Can i swap out my closed knuckle axle for j20 open knuckle axle?
Also can i reposition the leaf springs like the j20? I want to do this so i can add a new lift kit.
any guidance would be great.
Are the frames all the same?
thanks
new guy

[fulsizjeep]

I moved this to Modified FSJ Tech Section because that just made sense to me. Thanks -

[carnuck]

I was going to do that with my '73 J4000 The spring pads weld on easy enough but make sure the pinion is level or you will have a nasty vibe and steering issues.

[Stuka]

How good are your fab skills? The frames are very similar (really only holes and some brackets are different, oh and the cab mounts), so moving the springs under the frame is certainly possible, several have done it.

Is your J4000 currently 8 lug? If not, you will want the rear axle from the J20 as well. It will essentially bolt in, just gotta worry about brake lines and such.

[Tad]

It can be done, I did it SOA on a 72 J2000 but it could be done SUA and maintain the current ride height or add lift springs.

You will need spring hangers, 74-91 FSJ front springs, new brake lines, newer FSJ pitman arm, the correct u-bolts and spring plates, newer FSJ front shock towers, etc.
If using lift springs stock pitman arm and brake lines wont work after about 4" of lift and you might want to think about brake upgrades depending on your current brake system since this usually leads to bigger tires.

You will also need to be able to correctly weld and measure very accurately.
It helps if you have newer FSJ real close so you can mimic the measurements.

[carnuck]

The easier way (if your springs are still good) is put new spring pads on a newer axle with the pinion horizontal. Since they will go on "virgin tube" I suggest longer, beefier ones out of channel steel to help keep them from tearing loose or twisting. Use the newer steering parts and get the frame bracket to add power steering if yours doesn't have it. The box gets lowered down to a better alignment with the draglink. You may have to go with high steering arms to make it work.