Hello, I'm currently overhauling the soft top on my A124, I have parted out most of the cylinders for the roof out, and I'm going to replace the top and all the bolts in it.

But the bolt holding the largest cylinder is rounded, so I'm going to have to weld a large nut on it to pull it out. Do anyone know what material the MB bolt is made of?

hello,
I looked at mine over the weekend. I'm not exactly sure which cylinder you are having problems with. From inspecting mine, all the holding bolts/pins look like they are steel. you might try vise-grip pliers before welding in the car. Fires, burning wiring, damaging paint, and risk to interior requires special care for protecting everything when welding. if you post a picture, I may be able to help you further. Good luck!

We got it out by banging a larger umbraco into it, seems like there has been water in the car before, and the bolts had seized.

I did ruin the bolts, and from what I hear, you can get new ones from MB. But I did find someone I Germany that sold remade ones in stronger steel, because his roof collapsed due to wear on the bolts.
Feels good to do a complete overhaul