Flat / Countersunk Head
Direct Answer
Flat Head (Countersunk / CSK, standardized under ISO 7046 and DIN 965) screws feature a flat top face and a conical tapered underside (90° in ISO Metric, 82° in US Imperial, 100° in Aerospace). Designed to seat inside a matching chamfered hole, the head sits 100% flush with or slightly below the surface of the joined material.
Source: ISO 7046 / DIN 965 / ASME B18.6.3
Specs
| Standard Specifications | ISO 7046 / DIN 965 / ASME B18.6.3 |
| Head Top Profile | 100% Flat top face |
| Countersink Angle (ISO Metric / DIN) | 90 degrees |
| Countersink Angle (US Imperial / ASME) | 82 degrees |
| Countersink Angle (Aerospace / NAS) | 100 degrees (for thin structural aircraft skins) |
| Length Measurement Rule | Overall Length (Measured from top of head to screw tip) |
| Primary Applications | Woodworking, door hinges, machine slides, aircraft aerodynamic skin |
Critical Fastener Length Measurement Exception
In mechanical engineering, virtually all proud-head fasteners (pan, socket cap, hex, button) are measured by shank length only (from the flat underside of the head to the tip). Countersunk screws are the universal exception: because the entire head recesses into the material, a countersunk screw's stated length is the overall length including the head.
The 90° Metric vs 82° Imperial Angle Mismatch
- Metric (ISO/DIN) countersunk screws use a 90-degree cone.
- US Imperial (ASME/ANSI) countersunk screws use an 82-degree cone.
Driving an 82° imperial screw into a 90° metric countersunk hole (or vice-versa) causes point contact at the rim or throat, leaving the fastener cocked or proud of the surface.