Flange Bolt:

Flange bolt have a circular flange under the head that acts like a washer to distribute the load. They are non-serrated are sometimes called frame bolts. And usually used in vehicle frames, especially in trucks and anywhere a head bolt is required. Furthermore Flange bolt feature a nozzle, thereby speeding the assembly while eliminating the washer. They render four times the bearing area than of a standard hex screw.  Measure across the centre from one side of the internal hole to the opposite side of the hole. Because it measure from centre of the bolt hole to centre of the opposite bolt hole.

General:

Furthermore addition of steel washers to a fastener system is recommended to improve translation of torque input into bolt preload. Washers provide a smooth bearing surface, reducing friction between nut and flange. Generally this effect is greatest when washers are through-hardened. With a flange distributing pressure where the screw meets the surface, there is no need for a separate washer. Generally Extreme-Strength Steel Flanged Hex Head Screws.

To measure Flange Bolt: