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I'm currently reading about replacing the rear frame rail on my '69 fastback. The only part that needs replaced is the end piece, easily fixed with this patch:

http://www.metropartsmarket.com/catalog/image_classicmustang_c27_4280.html

However, what kind of welds would you use? I assume you need to weld it (lap) to the underside of the trunk drop offs, but how do you weld into the old frame rail section I'm keeping?

Thanks!!
David.
 

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You need to spot weld it to the trunk dropoff... drill small holes... 5/16 or so... doesn't really matter, but not too overly large. space them about every 2-3 inches along the "lip" of the new frame rail. do not just weld at the edge of that flange/lip thingie, it is not strong. you do need to lap weld the two sections of framerail together, though.
good luck, and practice welding upside down on something other than your car... it is totaly different than welding right side up!

- jason
 

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If you want a real "pro" instalation , do a hidden fishplate where the rail section join. You can also do a "diamond" cut at the sectionjoint. Either one will be vastly stronger than a butt weld and after grinding will leave no indication of any reair..
 

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Those frame rail extensions are pre flanged at the joint end and are designed for a lap weld. Obviously the best way to do a lap weld is to weld both sides. But if you're not replacing the trunk floor at the same time, you will not have access to the inside of the frame rail. Therefore a combination of lap welding and spot welding/plug welding will be necessary.

Once you've removed the old frame rail, you will need to drill 3 or 4 5/16" holes in the last inch or so of both sides of the remaining frame rail (the area the extension penetrates the original frame rail). You then insert the replacement frame rail and secure it by filing/plug welding the holes. Following the plug welding, you weld the lap seam.

Ken
 
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