Square Head Bolt, Steel, Grade 2, Zinc Plated, 5/16"-18 Thread Size, 1-3/8" Lg, 50 PK, Midwest Fastener, Mfr#: 09216

A square head bolt has a four-sided head. Compared to standard hex bolts, square bolts have a greater surface in contact with the part being fastened, and therefore provide greater resistance to loosening (though also greater resistance to tightening). Square bolts are typically mated with square-headed nuts.