they are termed a flange nut


Mark

Ok so I got the stuck one, front header rear stud out with a god old impact driver and a six sided socket. The whole thing came out clean as a whistle nut an' all. The other one was moving but tight half way out the stud sheared flush with the head

Anyway long story short. I got hold of a 3mm solid carbide drill bit and drilled down the middle of the old stud then went up to 5mm then 6.5mm then tapped them both M8. The carbide drill bit was brilliant went straight down the old stud. I think that that is the key to it. The carbide drill bit works so well that you don't need to apply too much force and so it doesn't wander off.

Finally back on the road

So did this solve your leaking header?

Oh Yes