Heads with Special Features

SQUARE Commonly found on cup bolts; also can be seen on raised countersunk (plow bolts/earth-moving) and hexagon (pump bolts).
The square is used to retain the head while the mating nut is tightened.
SHOULDER Normally round and normally associated with hexagon heads.
Often is used to allow a retained part of the assembly to rotate, eg: motor mower blades, tilta-door hinges.
Can also be used where a component in the assembly is made from crushable material which would not stand up to the tightened clamp force.
OVAL Usually associated with cup heads, agricultural and railway applications; also found on guardrail bolts.
Used as a retention device in a slotted hole.
WASHER RECESS Found usually on self-drilling building fasteners; used to retain a neoprene or plastic sealing washer.
Hardened serrated teeth ramped to bite into mating surface upon loosening.
Used in high vibration applications, particularly automotive; also used on nut products.
With teeth ramped in the opposite direction, will act as a paint or coating remover and give excellent electrical contact.
NIBS Less aggressive than serrated, used on smaller fasteners to scrape paint or surface coating to give improved electrical contact.
Often associated with mush or truss heads.
(High Grip)
A reverse thread applied to the shank just below the head allows a self-drilling building fastener to grip the crest of the sheeting material assisting the washer to achieve an improved seal.
COUNTERSINK RIBS Allows the screw to self-countersink the work piece on assembly, giving a flush finish.
Flower heads which have notches on the outer edge, similarly ream a seating hole, allowing them to self-embed.
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