Help!!!!!! Rounded bolt!

[fisherbear]

I'm trying to replace a thermostat and it has 2 10mm bolts holding it in.... I got one of them out no problem but the other one is rounded out bad... I've tried 4 different types of 10mm sockets, 2 different types of 10mm wrenches, adjustable pliers and I even went to autozone and got a set of the special grip tite sockets that are supposed to grab the bolt on the side and not on the edges... that one just spun around... I got a 3/8 socket and smashed it in with a hammer to see if that would get a better grip but that didn't do anything but spin freely...

my truck isn't overheating but when driving it yesterday it went well above the half mark to about 70% which is quite unusual for my truck (its usually around the 25%-30% mark) so it's more of a preventative thing... my 2 options are to screw in the good bolt and put in fresh coolant and pray it doesn't over heat in the texas sun.... or get this bolt out somehow... I'm sure there is some rust damage to the threads... I had the thermostat changed maybe 5 years ago... opinions? thoughts... thanks :?:

 

[Rearanger]

Vise Grip + Heat? There's probably a tool out there. Does not sound like a grade 8 bolt was used before.

 

[Will]

You could weld something to it. The heat of the weld would loosen it, too.