Can anyone think of a solution to my problem. When I built the car I got my uncle to thread some bar which I welded to the cahssis for the harness. Unfortunately he threaded it 12mm and not 7/16". The harness mount is now clamped instead of swivelling because top hats won't fit. It passed SVA fine but I'd like it sorting. Can't think of anything except reboring and perhaps a threaded insert but perhaps not too safe? I dunno. Any advice?

Drill out the top hats to 12mm.

But they will still clamp on the harness mount plate as opposed to sitting inside the plates hole. I can't really file the hole out without compromising the whole plate. The amount of metal around the hole isn't great enough.

That doesnt sound right, are you sure its 12mm as 7/16" is 11.11mm so you would only need to make the center hole in the top hats 0.89mm bigger?

Yeah it's my explanation, sorry. The harness plate that bolts to the car has a hole diameter that is just about 12mm and no wider. So even if I widen the top hat inner hole, the outer diameter will still be too large for the harness plate.

I get you know, the harness plates arent designed to use top hats, I just assumed they were like the ones I had which came with tophats.

Not sure then, the only thing you could do it would be to weld up the holes in the plates and redrill them to allow a tophat, I wouldnt recommend using a smaller bolt as 7/16" is the smallest legal size.