Well, I had some time last night to start cutting out the rusty portion of a subframe rail, and start brainstorming about how to most effectively repair it while keeping the underbody profile as close to stock as possible. Here's a photo essay of the work I got done last night. (having the body mounted on a rotisserie is going to make this job much easier).
The area you are looking at in the photo above is in the passenger's side footwell/firewall area. The mounting pad in the right hand side of the photo is for the front of the K-member. I wanted to leave the position of this mounting pad intact, so I could maintain the correct geometry when it's time to put it back together. The stud at the 11" mark on the tape measure is the rear mounting point for the K-member. I want to leave this in it's factory location as well. As you can see, the repair section needs to extend about 13" in order to re-form the "corner" of where the floor and firewall sections of the subframe rail will meet.
Here is a close-up of the front edge of the area that I cut out. This is immediately behind the front K-member mounting pad. You can see that it is just about 2-1/2" from the "hump" in what remains of the factory stamped rail, to the outside edge of where the factory rail used to be. I will be using 2-1/2" square tubing with 1/8" wall to rebuild this area.
You can see here that the factory rail was about 2-1/4" wide at this point. The replacement rail (2-1/2" square) will be slightly wider than the original factory rail. Since the original factory rail had a slight curvature side-to-side, the 2-1/2" square tube should work well to accommodate being welded to this slight curve.
This shot shows the slight amount of curvature present in the factory formed rail.
In order to strengthen the junction between the square tubing repair and the forward section of the existing formed rail, I will cut away part of the top and bottom of the square tubing, so it can slip over the existing rail, basically following the silver lines drawn on the factory rail. With some careful cutting and welding, this repair should be much stronger than the original construction, and will serve as a solid foundation for the rest of the floorpan repairs and eventual cage construction.
The rear section of the stamped subframe rail will be replaced in a similar manner, and will be welded to meet the forward section. The stamped steel "box" shown to the left will be either repaired or replaced with the same shape, but in 1/8" thick steel sheet.
That's all for now,
Steve.