using a 81 2wd crossmember on a 70

[GMC70c20]

will a 83 1/2 ton corssmember and upper cotrol arm bolt to my 3/4 frame without doing and mods to either?

[poorboy]

It should bolt up no problem. I've heard sometimes you have to drill 2 holes but otherwise it's bolt in.

[Dr. Drop]

Why go to all the work of removing the crossmembers just to change over to 1/2 ton suspension. Just take out your 3/4 ton stuff and replace it with the 1/2 ton stuff it all interchanges. Your 3/4 ton crossmember is the same as the 1/2 ton. Save yourself the time.

[GMC70c20]

you mean I can use the 1981 control arms with the 1970 crossmember? and does it matter if my 70 stuff is 3/4 ton and my 81 stuff is 1/2 ton?

[67Fleet]

that's correct., and no. Differences are ball joints and spindles as far as I've read.

[70Longhorn]

I put '88 R20 (crew cab) spindles and lower control arms on my '70. The differences I saw were:
1) balljoints - '88 balljoints will fit '70 a-arms and vice-versa, but must match the spindle of course.
2) Steering stops on lower arm - Although the difference could have been disc versus drum brakes.
3) Spindle dimensions - The vertical distance between upper and lower balljoints when attached to '88 spindles is larger than on the '70 spindles. I'm unsure of the actual measurement.
4) Cross member - Because of the spindle difference, I checked the a-arm pivot points at the chassis. The '88 had a larger vertical spacing here too.

So by putting '88 spindles on a '70 crossmember, I actually did change suspension curves. The roll center should have dropped, and it should produce greater camber gain. I doubt it'll make much difference though. It seems to drive fine even with a whacked-out alignment - I will fix that after drop spindles and bags go on it... soon.

I hope that is useful information.

[GMC70c20]

I'm not to worried about handling, just as long as it inst to terrible im more worried about it going straight. Is there shims inbetween the crossmember and frame rails? I wanted to take it off to blast and paint it but I dont know if it will just bolt up again or if I will have to do any kind of alighment.