Here is a copy of a post from the old forum I have saved from way back. It recalls his rear frame rail patching story. I forget the VMF'er who wrote it and don't know if he is still here. I have to patch mine also so please add any tips you come acrossed.
Looks like you got a good deal on the rails. I probably ended up paying
around $40 each for mine with shipping. There's a huge mustang roundup in
Bellevue, WA this July and I'll probably pick up some stuff cheap there too.
Well, here goes my frame rail experience. I've heard a lot of comments from
guys who did this and I think some even made a jig and took a million
measurements. When I scoped mine out, I just didn't see why all that was
necessary. When it was all said and done, there were only two measurement
that I really needed. I used to build fishing boats and some builders in
the shop just made everything look so difficult. This one guy had the
transom on his boat so marked up, it looked like he was drawing plans for a
house on the thing. Anyway, I'll try to keep it simple because really it
is. Most of the real 'work' is wrestling the old rails out.
Anyway, here goes. First, you'll want to start by loosening up the fuel
tank and removing it. It's easier get it out when the care is still on the
ground. Then jack the car up and remove the rear tires. I jacked it up
from the center of the axle housing and put my jack stands on the frame
rails around where the front leaf spring connection. Get it as high as you
possibly can. The more crawling room you have the better. Also block your
front wheels so when you let it down onto the stands, the car won't creep
forward and kick them out. Really make sure the car is solidly supported
before you go crawling under it. Get everything that you think you should
out of your way, ie: exhaust pipes, wiring, etc.
Then I removed the old shackles and let the springs hang down. Don't worry
about the rear end dropping, the shocks are still connected to the body and
will prevent that. Now that everything is out of the way, take a few
measurements. Measure where each existing rail is in relation to the fuel
tank flange. You really only need to measure where the rail is at the rear
most corner of the flange. The front side will be determined by the
existing frame. Then measure the new rail and mark where you want the new
rail to meet with the existing frame. The new rail is reduced where it
meets the old, so you have quite a bit of lee way here. Give or take a
quarter inch and you're fine. Double check all your measurements. It's so
much easier to correct a mistake on paper than on steel.
I used a sawzall to make the cut on the frame. I did have to cut a little
into the inner wheel well lip to get a good cut across the whole rail.
Maybe you have a better way, I just didn't care much about that. Then I
proceeded to find all the spot welds from inside the trunk. It's easier to
see them here than on the rusty rail, and I'd rather drill from above if I
can. I found that a small one and a half inch scraper worked well to scrape
the paint off so I could find the exact center of the weld. The more
accurately you can do this the better. Then I center punched all the holes
and drilled 1/8" holes first, then 3/8. Drill all the way through. You'll
fill them up with metal when you plug weld them. I'm guessing there were
about thirty or so spot welds to drill out on each side. There are also
four welds on the very back of the rail and two on the bottom where it
connects to the back end of the car. From here it's just a battle of will
power. Do whatever you need to yank, pry or beat the old rail out. Once
you get it out, you may find you need to patch a little metal in the trunk
floor due to rust you didn't know about. Now's a good time to do this.
Also, it's a good time to prep the rails and trunk floor. I used a die
grinder to rough up and expose bare metal where the welds would be. On the
new rails, I just roughed whole upper surface of the flanges that would be
taking the plug welds. You'll be doing all the welding standing inside the
trunk. All except the seam where the old and new meet. By the way, the old
rail is galvanized so make sure you have good ventilation. That stuff isn't
real healthy to weld on. A little won't hurt, just make sure you have some
Then, when I was trying to fit the new rail in, I realized that I had to
make two more cuts in the existing rail so I could slide the new one into
it. I just took the sawzall and made two lengthwise cuts at the bottom
corners of the existing frame. I cut back approximately what I thought the
reduced section of the new rail was. About two inches. Then I peeled that
bottom flap down and slipped the new rail up into the frame and pushed the
flap back up to hold it in place. You can weld those cuts up when you do
the seam later. The only critical thing here is to make sure the new rail
is pushed all the way back so that the upper rear part of it butts up into
rear support of the car. Doing this ensures the position of the rear
shackle tube is right where the old one was. Now just bump it around until
you get the rear part of the rail the proper distance from the fuel tank
flange like you measured previously. It was here that I used a jack and a
small piece of wood about a foot or so long to push the rail up so that it
was solidly contacting the trunk floor. I just tacked a few spots to make
sure it wouldn't go anywhere and then moved the jack to various places under
the frame and started plug welding everything together. I welded the seam
where the old meets the new last.
After both were done, I cleaned everything up and sprayed everything
underneath with paint. It's probably a good idea to put some good sealant
underneath where the frame flanges contact the trunk floor to keep any water
out of where you couldn't get paint.
From here just start putting everything back together. I had 13 hours into
my rails. I hope you get 'em done quicker. It's not a pretty job. I was
pretty filthy when I was done.
Mig welding is best for this job. Hope you have one. Flux core is OK but a
Hope this takes a little of the mystery away. I'm sure there's always a
better way to do things, this is just what I did. It seemed to work well
for me. The car looks good. Springs lined up fine.
Good luck in your project. I think the only thing I screwed up was that I
didn't have a six pack of good beer in the frig. That was a job that
merited some type of reward when finished. Get some beer.
I just thought of one other detail I'm sure you would've figured out, but
basically you're going to plug weld every place you drilled out a spot weld
which would include the 5 or 6 places where the very back of the rail fits
into the rear body panel of the car.
Also, since my car is probably more disassembled than yours it might take
you longer to prep it for this project. My rear valance, bumper, etc. were
already off. It will be obvious that these have to come off before you
start. My memory isn't the greatest, so I might have left off some little
details, but I think I covered all the bases.
As for welding, just get your heat right so that you get good penetration
with minimal burn through. With the trunk floor a little rusty (if the
rails were rusty, so is the floor) it's probably thinner than it used to be.
I hardly ever burned through the new rail. It was pretty thick. I burned
through the floor in some rusty places, but if you're careful you can build
it back up with short intermittant shots of wire. Just let it cool long
enough to get solid and shoot it again until you're back to metal again.
The closer together you can get the two pieces to be welded the less you'll
burn through. Make sure you're getting good penetration into the new rail.
If the heat isn't quite right, the metal will just sit on top without really
sticking well. It will look pretty good, but won't hold. A good weld will
be smooth and slightly raised. But with some rust it might not be so
pretty. It doesn't necessarily have to be pretty to be good. Either way
just grind off the tops if you like before you paint the floor again.
Good luck Adam and let me know how it all turns out.