3 years ago Old blue (1990 F150 4x4) I noted had spring hangers in the rear just about to go.
So I put her in the garage, blocked her up and proceeded to take it apart. What resulted took 2 months of weekends, but then again It was my truck so I went over and above.
New Rule: IF the spring hangers are bad, pull off the bumper and the bumper mounts because they are bad underneath.
For the rust to get in and take out the hangers, you have likely lost half the metal.
This last summer I scrapped Ole Blue and this weekend I scrapped the rear end to get the tanks and wires etc for 'New Blue'. I took a pic of how the frame on old blue looked when I got her apart 3 years ago, this is after removing all the rust, PORing it, brushing it, and repainting it - the stuff does not hold up!
You are going to find that the metal under the hangers is thin, and the metal under the bumper mounts is thinner. I had a huge class 4 under there, so I was close to having it pull out. Too many people make the mistake of towing with the bumper, look at the pics to see what the metal is like once the rust is removed.
The next pic is the solution I came up with.
I sandwiched the frame and made my own rear crossmember. The result was far stronger than the frame as it came from ford, AND it used the hitch as a stressed member.
Early on, reese, draw tite etc made the mistake of telling everyone to drill the 2-3 holes per rail and mount to that. when I got the truck I drilled out the rear cross member rivet and used that as the first bolt. It moves the hitch perhaps 1 inch forward, but it increases the amount of vertical weight the hitch can take and since it now couples with the frame rail, there is less tendency to turn over the rail when doing a braking turn.
The hardware is 7/16's grade 8 that go thru the inner plate, frame, outer plate and hanger.to cover any spaces between the bottom of the sandwiches, I used grade 8 washers. all hitch hardware was upgraded to 9/16's grade 8. Why are the inner sandwiches so long? they extend up to the next crossmember to stiffen the weakened side walls and bolt thru every large hole with large hardware.
I used regular steel, scrap from a machine shop, 3/16ths thick. the cross member was made from thick angle iron, with the plated section made from 3/16ths braked twice to give the stock bend, and as you can see, I cut out for the tank straps.
If anyone is interested in this fix, you are more than welcome to stop by the shop or write and I will see if I cannot work up some engineering diagrams.
Most of those 2 months were spent making the plates. Actual re-assembly took 2 days.
About an hour after these photos were taken, I took a sawzall and cut the entire rear of old blues frame off and chucked it on the neighbors scrap heap.
Im down to an engine and tranny between the front rails.
I have an '87 F150 with rusty frame rails just behind the rear wheels. (You know where all the crud from the tires gets blown up into.)
I saw your frame rail fix and I was thinking of doing something similar.
How thick is the material you are using for the side members? I was thinking of using 3/16" and just bolting them to the inside of the frame member. This way I can attach the rear spring hangers with 7/16" Grade 8 bolts, nuts, and washers.
I plan on using Rust - Mort on all the rust and sealing it with some Rustoleum Black. The Rust - Mort will return the rusty metal to its base metal form and the Rustoleum should seal it from other weather...at least for several years.
My rear fuel tank is a rusty mess along with the tank straps. Those are all being replaced with new.
Any further suggestions or direction is appreciated.