I have some rot in the round tube steel cross member between frame rails near the rear of the gas tank. This crossmember supports a bunch of stuff like the upper suspension links, gas tank, etc. Mine is rotten towards the middle and has developed a crack. What I want to do is just slide a smaller diameter tube right into the existing tube, throw a couple welds in there, and call it a day. The existing tube is not in the worst of shape, it is just really banged up in the middle by the crack. I searched and was surprised that this repair has not been discussed before. Has anyone else had to do something like this?

You should take a picture and upload it so we can see and tell what can work or not.

@brillo_76 has done it. He's the man on fixing up these frames. I believe he has done just as you mention. Check out his build section.

Yup use a black steel pipe 43 inches long with a close to 2inch outer diameter. I clean the old cross member tube out. then coat it with FF and slide a new one inside. Once that done. I weld it on the ends and then Weld the brakets to it in the thin spots. As brackets can be welded to the new inner pipe super easily.

You mean like a regular schedule 40 black steel pipe for plumbing natural gas? Is the 2” black steel pipe a nice tight fit?

Yes... Its slightly smaller but welds in greatly.. being the rust will swell. I use black pipe like natural gas plumbing 1/1/2 inner diameter... which is close to 1/4 ich thick is super close to 2 inch outer diameter slightly less then that but close.

IF you have find a pipe a little closer that steel it will work great. This is the closet pipe that i came across easy to get ahold of that works. :-)

Right on thanks! From my rough measurement I will need about a 2-1/16" outer diameter pipe. I may actually hit up a fabrication shop this weekend. I am gonna pull into the parking lot, take a rear tire off, and just throw pipes in there until I find one that fits the best!

The chances of you getting a 2 and 1/16 OD pipe to go through a rusted tube cross member is extremely unlikely.

I actually had to pound my pipe in with a sledge to get it in as they rust and rust expands the rotting steel.. hence why I dropped the size slightly.

I ended up getting a piece of 2" outer diameter steel pipe from a local steel supplier. Are you ready for this......the walls of the tube are 1/2" thick! They had it laying around and I paid $30 for it. Like you said the frame was in bad shape inside and there is no way it was simply going to go through. So I reamed out the frame with a 2" diameter hole saw to clean up the rust inside. Then I greased up the new pipe and rammed it through. I also had to use a sledge to get it through. I didn't have to pound on it as hard as I could but it definitely needed some force. I put a couple small welds on the end, probably didn't even have to do that.

So in the end a 2" outer diameter pipe would probably go right through if the existing frame wasn't already that damaged from rusting on the inside.