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Glossary
Underhead Fillet
An underhead fillet is the rounded radius transition between the bearing surface under a fastener head (bolt/screw head) and the shank. Instead of a sharp corner, the underhead area is blended with a radius.
In fastener terms, it matters because that radius:
- Reduces stress concentration at the head-to-shank junction (a common fatigue/crack initiation area)
- Can increase fatigue life in cyclic-loading applications
- Affects fit and seating: if the mating part has a sharp edge or a tight counterbore/spotface, an oversized underhead fillet can interfere and prevent the head from fully seating (sometimes addressed with a chamfer, washer, or specifying a reduced fillet)
You’ll commonly see it controlled by a specified “underhead radius/fillet radius” in fastener standards and drawings, especially for structural bolts, flange bolts, and socket head cap screws.