found some underbody rust around my subframe support bracket on my w115. what else do these bolts go through besides floor pans?

The title of your JPG says it all.

The bolts don't go through anything else. They rely on the integrity of the floor pan to maintain proper suspension alignment.

Time for new floor pans at a minimum.
Check around for other key rusted areas (jack points, trunk deck, front frame rails, inner fender wells, etc.). You might be frightened at what you find. Remember, these are "Uni-body" construction vehicles and rely on chassis components to provide strength.

http://www.peachparts.com/shopforum/body-repair/339888-w114-rocker-panel-repair.html

http://www.peachparts.com/shopforum/body-repair/232688-rear-fenderwell-rust.html

http://www.peachparts.com/shopforum/vintage-mercedes/229683-leg-bones-connected-knee-bone.html

http://www.peachparts.com/shopforum/vintage-mercedes/205060-kiss-death-all-old-mercedes.html

http://www.peachparts.com/shopforum/vintage-mercedes/183947-rust-never-sleeps.html

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Just goes into the floor pan - but the whole pan needs to be stiff and rust free which is often a bit difficult for an old Mercedes!

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

Hah! Well done Mike - super fast posting!

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

Yeah, I'm bored and have nothing better to do today than surf the 'net.

Drew up a crank trigger wheel for the M115 TBI conversion and am sending the drawing off to have a custom one made. Gonna be a couple of days before it's done so, for the moment, the wife's Benz is sitting in my work area blocking any projects I had planned.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Okay cool i was planning on replacing soon going to tack in new ones and just cut out and most likely fiber glass the rust on the underbody

Bear in mind the stiffeners were put there for a reason when you're smearing fibre glass where once there was metal. If the floor pan is not stiff enough the subframe mounts at the front will probably end up bending at the higher part in the chassis that is in a closed box section which makes it very awkward to repair...

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

I meant putting fibre glass on the non structural part of the car like the very bottom of the car under floorpans

The entire floor pan is structural. Using fiberglass to repair pin holes is fine. You should not replace an entire section with fiberglass mesh.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

If you are unable to get metal clean enough to weld but the metal is otherwise strong. On various cars I've used 1/4" bolts to distribute loads and welded where I could.

An example would be a domestic ( USA ) car with a pinch welded lower rocker panel. Weld the top, bolt through the pinch weld.