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Who says that the box section here is 4mm 223?? where did you get that figure from because I cant see it anywhere on the original post.

In any case, welding 12 mm to 3 mm with a weld length of...

Yeah you can but they are only 8mm thick.

I always go for 12mm. They little extra it costs is worth it for piece of mind.

If its only going to pull 900 kg that coupling is probably fine as long as it isnt rusty inside.

And for your info the welding on a trailer under 2.5 ton doesn't have to be certified. It just has...