190SL Undercarriage Welding
The old pitted floor pans were stripped and new pans welded while the car was on jack stands on the floor to avoid distortion of the chassis. The chassis/frame was in superb condition except for on or two places, which required local repairs. The floor pans consist of four separate panels factory spot welded to the chassis 'n' shaped side and cross members from beneath the car. This forms a box section which was not sealed, primed or treated by the Mercedes-Benz amd results in rust from the inside out. The replacement floor pans are thickers than the original and are two sided zinc coated steel. The zinc coating creates a barrier against rust and can be painted over once prepared well. All chassis members should be checked carefully by tapping with a pick hammer to ensure that they are not weak, especially the trailing arm supports which could become critical if the trailing arms were to break loose at high speed.
The SIP 170 turbo Mig welder performed nicely but welding on you back with overhead plug welds took time mastering. Too low a speed and rating on hte Mig welder and the weld would not hold up. Too fast a wire speed and too high temparature the weld will fall out, hopefully missing your arm. Precautionary measures should include full body and head protection, safety first. I sort the advice from a Hot Rod forum, HotRodders, and this was the response by one of the members:
"Remember to work the pool. If the weld is falling out of the hole, two things to look for here.
1) You probably actually have too much heat or moving too slow and the weld is molten to the point that it is sagging under its own weight. Gravity is getting you there.
2) Also make sure that there are no gaps between the 2 pices of metal. If this happens, it not only is a bad seal, but it makes it a bear to weld vertical plug welds.
If the tacks are not holding, again it is a missed location of weld or lack of penetration. A lack of penetration may be from weld technique or heat. I really doubt that it is heat since we are talking relatively thin sheetmetal here. When doing that type of plug weld, I usually try to start in the middle of the hole and work the weld out to the edges while circling around the entire inside of the hole. If you see the molten pool starting to sag, stop for about 3 seconds and hold the torch in the same spot. When it is too dark to see the pool through your helmet, hit it again and finish the weld.
Overheald welding is probably the most difficult if that is what you are doing. If you take two pieces of sheetmetal about the same guage, set them together on the ground or bench, you can tune your welder comfortably before trying to overhead weld the floor pan. Look at the backside of the second piece of scrap. You should see where the heat has penetrated through it. Not fused material like a pimple, but just enough to discolor the metal outlining the weld from above."
Armed with this advice and with several practice runs I mastered the overhead plug welding.
While the car was on jack stands I built a trolley. This was rigged with two protrusions to one side clear of the body so as to provide a pivot point when rolling the body over its side. Next process is to complete the grinding, de-rusting and preparation for paint. Much easier than working under the car on your back.
Time now to attend to the right hand wheel well, which was previously badly repaired by plug welding rust holes. The lower portion was cut cut out. An new repair panel was fabricated complete with drain hole. This was butt welded an finished off by lead loading.
