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Discussion Starter #1
I started to change the leaf springs and rear shocks when I found rust. If anyone with experience can give their opinion on these questions I would appreciate it.

To fix this, will I need to cut off the frame rail before the rust and then weld a new frame rail in place?
If so, how do I join the two frame rails?
How far forward do I need to cut it? - refer to the second picture to show the non-rusted part further towards the front of the car.
Do I need to cut out part of the trunk to fix this?
What general steps you would take if it was your car?

Thank you! Here are the pictures.



 

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1966 Mustang Hardtop 289 4 Speed
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You are going to need the frame rail repair piece and at least part of the rear trunk support and possibly some of the trunk panel by the look of things. Buy the trunk panel and drop off section just in case, it's not that expensive. Buy your panels from NPD or Virginia Mustang. Build yourself a jig that lines up with your rear spring attachment point and you should be good to go. Be prepared for things to snowball a bit, but overall it shouldn't be too bad of a fix if you already have a mig welder.
 

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That's actually not to bad. They sell a short section of rear frame rail to do just this job. It will fit inside the old rail where it will join together. You can add a couple of plug welds there as well as welding it through your old spot welds after you drill them out to remove the rear frame section.

The part the rail kind if sits on is rusted away on the outside edge near the trunk drop. You can try to fab a piece or buy the entire piece and just cut off what you need from the new part and weld it in.
 

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Discussion Starter #5
You are going to need the frame rail repair piece and at least part of the rear trunk support and possibly some of the trunk panel by the look of things. Buy the trunk panel and drop off section just in case, it's not that expensive. Buy your panels from NPD or Virginia Mustang. Build yourself a jig that lines up with your rear spring attachment point and you should be good to go. Be prepared for things to snowball a bit, but overall it shouldn't be too bad of a fix if you already have a mig welder.
Thank you for your help. So, to make sure I have this right:

I should get this rear frame rail: https://www.npdlink.com/product/rear-frame-rail-canadian-made-28-inch-long/100799
Trunk drop off: https://www.npdlink.com/product/trunk-drop-off-panel-canadian-made-035-inch-thick/101032
Rear trunk support - I don't know what that is, could it be this: https://www.npdlink.com/product/rear-floor-cross-member-repro/100071

Build yourself a jig that lines up with your rear spring attachment point
Is this so that I make sure to get the frame rail into the right location?
 

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Discussion Starter #6
That's actually not to bad. They sell a short section of rear frame rail to do just this job. It will fit inside the old rail where it will join together. You can add a couple of plug welds there as well as welding it through your old spot welds after you drill them out to remove the rear frame section.
Ahh, OK. So first I get a new frame rail (here) and then I will cut out the old one based on the length of the new one so that the end of the new one slides into the existing. Add a couple of plug welds and it will be secure. Is that right?

The part the rail kind if sits on is rusted away on the outside edge near the trunk drop. You can try to fab a piece or buy the entire piece and just cut off what you need from the new part and weld it in.
Any idea what that piece is called? Is it this by any chance? https://www.npdlink.com/product/rear-floor-cross-member-repro/100071

Thank you for your help!
 

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Ahh, OK. So first I get a new frame rail (here) and then I will cut out the old one based on the length of the new one so that the end of the new one slides into the existing. Add a couple of plug welds and it will be secure. Is that right?



Any idea what that piece is called? Is it this by any chance? https://www.npdlink.com/product/rear-floor-cross-member-repro/100071

Thank you for your help!
Yes that's the piece that fits along the back.

I would drill out the spot welds along the trunk floor that hold the frame rail section in place and reweld the new frame piece via the old spot weld holes you drilled out. Or you could cut and grind all the welds from underneath without drilling them from the top and stitch weld it all underneath. I would drill out the spot welds myself. Plus weld the plug holes where the two frames join.

What i also suggest is to measure everything from side to side and from the rail to the floor. What I like to do is measure from the inside of both bushing flanges and write that number down. I only do one side at a time so that I can reference that measure to locate the new frame rail section. Measure to that same flange opening from the floor. I like to mark the floor where I measured it from so I repeat the same measurement each time. You could drop a plumbob line too. Just make sure you get the new piece in the same spot.
 

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Discussion Starter #10
Yes that's the piece that fits along the back.

I would drill out the spot welds along the trunk floor that hold the frame rail section in place and reweld the new frame piece via the old spot weld holes you drilled out. Or you could cut and grind all the welds from underneath without drilling them from the top and stitch weld it all underneath. I would drill out the spot welds myself. Plus weld the plug holes where the two frames join.

What i also suggest is to measure everything from side to side and from the rail to the floor. What I like to do is measure from the inside of both bushing flanges and write that number down. I only do one side at a time so that I can reference that measure to locate the new frame rail section. Measure to that same flange opening from the floor. I like to mark the floor where I measured it from so I repeat the same measurement each time. You could drop a plumbob line too. Just make sure you get the new piece in the same spot.
Awesome. Thank you for the help! I will get started with measurements and taking everything apart.
 

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What the heck is that Bug to the right of the frame rail...???! It looks like a million year old extinct Trilobite!

Yeah, You need someone to cut out the old Frame rails and weld in some new replacement ones...

:eek:)

Tony K.
 

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Discussion Starter #12
What the heck is that Bug to the right of the frame rail...???! It looks like a million year old extinct Trilobite!

Yeah, You need someone to cut out the old Frame rails and weld in some new replacement ones...

:eek:)

Tony K.
Haha! It is a praying mantis egg sac. I like those guys, but this one will be cleaned off.
 

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Cut down about half way then right angle the cut horizontally for say 3 inches or so then back down to cut the rest off. It makes a notch in the frame rail. You would cut the matching notch for the replacement piece so they fit together. Weld it inside and out. For extra support weld steel inside the frame rail over the notched seams with something like 8 inch steel plate on the bottom and both sides. That is how I would do it anyway.
 

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Not to HiJack your thread, but the kids are nearly raised and it's time to start back on my first car, 70 Mustang

It's been sitting for 28 years and I was young and did my floor boards and stripped the car back then. I hired someone to put on my quarters as I was scared of the major body work but it never was completed and brought it home.

My 70 has bad rear rails as well and I bought the American Designers rails, (not primered back then) but figured I had to have a jig or something to ensure it is square with the world and didn't want it to be dog tracking.

I just put in a lift and it seems by reading this that it shouldn't be overly difficult to do this task myself.

Good luck to ya, I'm going to start planning mine.
 

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Discussion Starter #17
My 70 has bad rear rails as well and I bought the American Designers rails, (not primered back then) but figured I had to have a jig or something to ensure it is square with the world and didn't want it to be dog tracking.

I just put in a lift and it seems by reading this that it shouldn't be overly difficult to do this task myself.

Good luck to ya, I'm going to start planning mine.
Super jealous of you for having a lift! Good luck to you as well. I think this will be quite a project for me, but I agree, these guys make it sound easy. We will see.
 
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