At some point in every car build, I believe everyone comes across a task that they dread having to do. For me, it was addressing the rusty lower rear frame rails. I tried ignoring them, but every time I took a photo of the rear end, it seems the camera was determined to remind me that something had to be done:
Peering down inside the rear frame horn was a horror show. The little tower way down inside the rail is the captured nut that the aft engine cradle mounts to. It clearly wasn't going to hold much load at this point.
Part of my reluctance to deal with the problem was that the trunk sheet metal obstructed access to the inboard side of the rail. I hadn't wanted to remove any more of the trunk than necessary, but I gave in and broke out the cutting wheel and let 'er rip:
I was careful to cut the trunk only where necessary in case I wanted to re-use it at some later point, but I realized the exhaust system would have to occupy this area when all is said and done. The lower half of the trunk sheet metal isn't attached to anything except the sheet metal trunk walls above it, so no structural rigidity was lost.
From this top view you can see why the lower half of the trunk pretty much needs to go to make way for exhaust plumbing. Even though some guys with Northstars in their stock-bodied cars have kept most of the trunk, the difference here is that there's less rear overhang on the F355 than on the Fiero:
Here's a rear view showing that the engine cradle mounts to the corroded portion of the lower rails:
Obviously with the rust extending to the aft cradle mount, I needed to remove the engine cradle and powertrain assembly to fix the rails. Seeing the car on all fours was fun while it lasted, but it was short lived.
The good news was that when a friend of mine heard of my dilemma, he offered me a set of pristine rear frame rails recently cut off his own car. Manna from heaven! Within a matter of days, they were sitting in my shop waiting to be assigned a new life:
Having Graham's rail ends on-hand made it easy for me to decide just how much of my own rails I should replace, and how to go about doing it. The rails are made of two "Z" sections spot welded together along two flanges forming a box. So I got busy drilling out the 30 or so spot welds holding the two halves together on my car, and slicing up vertically to remove the entire inner wall and floor section:
The photo above is after I'd cleaned up the rust with a wire wheel. The outside wall is pitted somewhat but not badly enough to have made me want to replace it. The real damage was to the inboard wall and floor section:
Here's a close up of the area where the captured nut for the rear cradle mount used to be... just a mess of rust between the mount doubler and the sheet metal floor: