Gentlemen,

Yes, you read the title right - I intend to take the bed off my 1979 F250 (2wd, auto, 351M), disconnect all drive train from the frame up to the transmission, and cut the rear ~5-feet of frame right off, then rebuild it.

The existing frame actually snapped on me a few months ago because it's so rusted. The front half of the frame is mint. Go figure. And I had to take the bed off anyway 'cause it's also a rust bucket - to be replaced.

So.....I'd be real appreciative of any pearls of wisdom, things to avoid I haven't thoguht of, etc etc. Here's what I've got in mind:

I'm going to replace the frame rail with a strait rail. No more hump over the rear axle. If I align the new frame to the straight section of the original, then I figure no hump will reduce frame-to-axle clearance by only about an inch. I'll install short axle bumpers. The alternative is to raise the frame on the leaf spring mounts to maintain axle clearance, but then I'll have a one-inch rear lift.

There'll be about 3-feet of overlap of the old rails and the new. I'll bolt them together at the extremes, and pre-drill holes in each that don't line up, then weld the holes to the other rail. The new rails will fit over the old ones, so the flanges of the U can then be welded as well. No vertical welds.

I'll probably have to fabricate everything - leaf spring shackles, gas tank hangers, cross members...

I expect that getting everything straight and true and square is going to be a nightmare, as I'll be doing this outside in my gravel driveway (hey, I changed the motor out there, can this be worse?)

While the box is off, it sounds like a good idea to finally run dual 3" exhaust pipes.

Gas tank - right now I have just the front tank, under teh left side of the bed. The rear tank was removed ages ago. It will interfere with dual exhaust pipes. A centre-mounted rear tank would solve that, but it always seemed silly to me to hang all that weight cantelevered off the rear of the truck, reducing useful carrying capacity by that much.

Well, there you go. Anyone attempted something this nuts and lived to tell the tale?

 

All I can say is that you are a far braver man than I.
Good luck. Take pictures.

 

Since yesterday's post I 've been reading about boxing frames - might be a better idea to build strength over top of existing rather than having to align everything new....hmmmm....

 

Thats kind of the way my frame is, front half is like new, the rear half is still structurally sound, but smooth as curdled yogurt

Do you have any pics? When you say "snapped", I'm imagining it breaking liek a twig and you now have 2 halves of frame. . .or in "snapping", did it just crack and bend at its weakest point?

I wouldn't ever think of patching this frame if its as bad as you make it out to be. You'll be patching good, strong steel to weak, rotten steel which will just transfer breaking stresses to the end of the good steel cuasing a possible failure there. Plus, you will have to get rid of ALL rot, or it will eventually destroy your patch work.

I'd feel safer either grafting on a better section of truck frame from another truck, or straight rail it like your thinking. Though, I'd need to sit down with a tape and note pad while I look at my frame before I could even think of any ideas though to toss at you

Best of luck!

P.S. Or bite the bullet and find a good donor for the frame and just swap over. . .maybe paint the donor frame too

 

Ya, the more I think about it, the more grafting teh straight rail onto as much o fthe old rail as possible sounds like the best option. I should be able to leave old rail almost back to the the axle. There's be a good 4 or 5 feet of original frame rail. Then I'll fit a slightly larger and heavier guage U-section over top of that and extend it back to where it's supposed to be, and all the way forward to the cab. With a half dozen grade 8 bolts and another half dozen through-hole welds, plus welds where the U-frlanges parallel each other, it's not going to be going anywhere.

 

Welder skill and weld quality are key with this type of rebuild. I know some self taught welders that have gotten themselves in real trouble with poor quality welding. Choosing the right process (mig, stick, tig) is also important. Make sure the metal is VERY clean of all rust and contaminants. Use amperage guides for the metal thickness and ask a professional for help. Find a local welding instructor or certified welding inspector for advice. You will have to use some additional bracing as butt welds on a frame will be too weak alone. It can be done, but I would not want your newly welded frame to “snap” on you as well the first time out.

 

Thanks for the advice - but no butt welds here.

The inner dimensions of the U of the new frame rail will be equal to the outer dimensions of the old frame rails so that the new one slips over top of the old ones. They'll overlap a good 4-feet. In the overlap area, there'll be holes that align for bolting, holes that don't align for through-hole welding, and I'll weld along the flanges. That should be tons and tons of reinforcing.

But, ya, I was seriously consdiering getting in one of those mobile welding services to do the welding. There'll be spring mounts and cross-members to weld and everything...

 

Sounds like that should work great. Those mobile guys will charge at least $40 an hour including travel. Stick or mig will work fine, just don't use a 110v mig welder. They do not have adequate heat to get the penetration needed for frame welding.