Rectangular tube crossmember

[werewolfx13]

My stock transmission cross member is twisted in my '76 k20, I might be able to heat it up and twist it back to get it bolted back in, but I was just thinking about making my own crossmember. I was thinking of using 1/2" steel plate bolted to the outside edge of the frame with 2"x3" 1/4" wall tube steel welded to the plates for the actual crossmember. With adequate clearance, 1/4" angle iron welded to the inside edge of the plate and bolted to the bottom edge of the frame. I could use just about any poly or rubber trans mounts and make the rest of the crossmember to fit. I have most of the steel stock I would need, and I'd rather not crawl through the muck to try to find a crossmember at the junkyard. Thoughts?

[Engineer]

Should work as long as you don't change your u-joint angles very much.

You don't want to build in a vibration.

[bd]

The frame needs to be able to flex.  Plan your design so you don't make the frame rail at the crossmember attachment too rigid or you may concentrate stress and eventually develop cracks.

[werewolfx13]

I hadn't thought about the frame flex, the materials I'd be working with don't have Flex in their vocabulary. I'll try tweaking the old crossmember back into shape first. A pipe wrench and cheater pipe will do the job if I can lock the crossmember stationary while I twist it.

[rustythe4x4]

I realize this is an old post, but attempting to straighten your crossmember will likely result in failure.  Best bet is to find a donor truck and scab the member from it.
I know guys that have built their own with no ill effects to the frame, and I have built my own as well between the spring hangers.  Keep an eye for cracks, but unless you abuse your truck regularly, I doubt you will have an issue.

[werewolfx13]

I've long since put the old one back in, I didn't straighten it, just manipulated it as I put it back. Was slightly tedious, but went in without a problem. Bolt holes were only off by half the bolt diameters,so even when we loosened the trans mounts to put the t-case back in, everything went back together fine. Biggest bear was replacing the old bolts going through the top of the frame rail..ended up re-using one of em,. managed to loosen the body mounts and jack the cab up JUST enough to drop a 3/4" long x 7/16 grade 8 bolt in one of the holes, the other one had a 1.25" bolt in it and I couldn't jack the cab up enough to clear it and get it out.