Rust around rear subframe mount

[Layne]

Hard to see I know, but here's what I'm dealing with: There's a thick plate under the rear seat that reinforces the subframe mounting area. This plate has some space under it, between the plate and the body of the car. Somehow rust got started in this space and completely ate away the body beneath the plate. What you're seeing in the pic is mostly the plate (covered in rust, but not really damaged due to its thickness). From above everything looks totally normal, but if you poke something under the plate, it goes through to the outside. The plate is still securely welded around the rust hole, as well as the wheel well and side of the car, so it doesn't seem like anything is in danger of collapsing. The question is do I attempt to remove the reinforcing plate to repair the hole, and then reinstall it? Seems like a fairly tall order, it has many spot welds and some beads on the edges. But repairing the floor with the plate still in place seems very tough too and less likely to get all the rust out (maybe not a big problem without continued water exposure). What have others done for this problem?

 

[eriknetherlands]

That hurts ..
how much spare time do you have, and how bad do you want to drive?

Exactly such a rust spot is what lead to the demise of my spare time over the past 2 years as well. In the end I opted for the removal of the reinforcing plate. However as that holds the large pin, and thus the subframe, alignment is key. I build a frame inside the car to reposition it in 3D where it was supposed to go back.
check my thread;
http://www.e9coupe.com/forum/threads/fixing-that-little-rustspot.18521/

It seems that also your inner sill (vertical, where it meets the floor) has a hole. If your sills haven't been repaired in the past, I take a guess that the inner sill structure is gone as well.
Mind you that this location is taking roughly 1/3rd of the pulling forces of the rear axle. The other 1/3rd is on the other side, and the rest is left to the differentail mount. Imagine driving over a speed bump and this brown lunp fails? You'll have a two wheeler before you can find a swear word....
Now probably it's still is roadworth, so it's not that critical. But i would take some time to find out the extent of the rust.

You can access the inner sill from the inside, to get a view just behind the vertical bit of the inner sill behind the shiny pin. See my thread for into on that in post #71 (page 4)

Erik.

 

[Layne]

Thanks! There's some very helpful pictures there.

This is a paying job, so I have plenty of time, but my customer really wants to drive. He isn't hung up on perfection, but having the subframe not fall off would definitely be appreciated.

I noticed the rusty floor panel folds upwards against the sidewall of the car, essentially sandwiched between the reinforcing plate and the sidewall, so no hope of a proper fix without removing the plate. So I've set out to do that.

Yes, there is rust in the rear part of the sill. It doesn't seem that awful. The real problem is that the car has already been painted. The painter just welded up the external holes and did not dig deeper. So I can't do much without ruining the paint. One unexpected salvation: it's actually 2 cars welded together. So while the back half has less than average rust, the front has virtually none at all.