I know sods law states that there's always one bolt/nut/screw that will be seized and a b**ch to remove. Well the law hasn't failed me today. I'm trying to replace the (plastic) valve cover gasket on our focus zetec se. Got 11 of the 8mm stud bolts outs easily but the 12th is seized. I've tried locking a couple of nuts onto the thread but that didn't help. I've got the plusgaz soaking in now but wondering if anyone's on here's got any tips. I'm thinking of welding a nut on next but I'm worried I might shear the stud if I use too much force.

plusgas and heat, work it a little back and forth if you can. Your right, too much force will snap it.

The action of welding a nut on is apparently a good way to crack rust. If you attach the ground lead on the engine rather than on the stud, the power has to conduct itself through the rust. With rust being slightly higher (?) resistance than metal it heats up and cracks.

Never tried it myself, that was something from a kit car magazine. Personally plusgas has never failed me, the stuff is amazing.

[Edited on 4/1/13 by Slimy38]

Have you put the other bolts back in and done them up reasonably tight before attempting to remove it? I find sometimes if all the other bolts are out the the one left is sometimes pinched as the item is not sitting flat. Worked for me on a lot of occasions.

Also try tightening it a little to break the rust. Dont over do it though.

[Edited on 4/1/13 by Alfa145]

Do it with the engine hot, and nip it up, then try and undo it?

Had this happen on a Silvertop. Had to grind the bolt head away with a Dremel to remove the cam cover and then luckily the remains just screwed out.

Also before attempting to move it whack the head with hammer, helps break the stiction where it has been fixed for years.

I have also found the two nut method only works some times and usually there isn't enough room to get a proper stud remover in, stud removers are also only work sometimes.
One trick I have used with studs is a variation on the two nut method using a drop of permanent grade Loctite on the threads and giving enough to cure.

Well, I left it for half an hour, cam back & Plusgas had worked it's magic, thanks for the replies though guys.