Fitting standard rear bumper

[RayW]

Hi guys
 
As above I have got a Mazda OEM FD3S rear bumper to fit onto my car.  The bumper has three threaded studs either side of the bumper to attach onto the rear quarter panels,  anyone know the sizes of the nuts required?
 
I\'m planning to get some Nylon lock nuts, but if standard is easier then I\'ll get them.  As you can imagine it\'s not easy trying to lug a bumper down to the local hardware store to measure up :Giggle
 
Any info will be good
 
Thanks
Ray