8mm (5/16") standard hexagon washer head fastener.
Fasteners in this range are all fully threaded. See fastener specifications link below,
These fasteners have a minimum 500hr SST coating. Link below.
When fixing through two or more steel sheets, with gaps/spaces, remember to predrill upper layers to prevent jacking problems.
Use a purpose made screwgun for installation. Cordless drivers and drill drivers are ideal with torque set fairly high on a high speed and in driver mode. Please note: Impact drivers should not be used under any circumstance and automatically voids any warranties.
Figures throughout will be ultimate test results with no safety factors.