Can anyone tell me if these are different to inlet manifold studs.

Ford supplied me with the same studs for inlet and exhaust manifold but the first manifold stud i tried to torque up suddenly went very loose and i managed to twist the stud in half.

I had no problems with the inlet studs using copper nuts but the exhaust nuts look like a stronger metal.

Could this just be a naff stud?

Any help or advice would be great.

Cheers