FD seat belt bolt size

[dap1977]

HI,
Does anyone know what size thread the bolt is that secures the front seat belts to the floor underneath the front seats?
It looks like an M11 but I am unsure of the pitch. It looked like a 1mm pitch, but I purchased 1mm and 1.25mm pitch taps to be safe and it still doesn\'t look right when compared to the bolt.
I need to clean the thread up in the floor as its full of debris.

[boosted]

m10 x1.5 metric fine. 99%
7/16 unf 1%

[dap1977]

Turns out it is 7/16 UNF-20 thread.
 
Thanks for you help.