Seat to Floor bolt?

[standups]

what is the thread on the bolt that bolts the seat to the floor??

[RallyBob]

Umm....metric?

I think it's the normal 8 mm x 1.25 pitch, but the odd thing is the head diameter of the bolt. It's 11 mm instead of the typical 13 mm.

[GTJIM]

Yes, standard metric thread size - it is the hex head which is a special smaller size so the smaller socket will fit around it and between the seat rails.

If you cannot find bolts with the smaller 11mm hex head use Allen Head cap screws to make the job easier. Don't use bolts that are too long as the threads will poke through under the car, get wet and RUST solid. It is also a good idea to coat the threads with Never-Seize or some other anti-seize compound to make them easier to undo next time.
Even consider using stainless steel ones if you can find them!