Counterbore vs countersink (and spotface)
The difference between a counterbore, a countersink and a spotface, which fastener each suits, the standard countersink angles, and how to call them out on a drawing.
A countersink is a cone-shaped recess that lets a flat-head screw sit flush with the surface. A counterbore is a flat-bottomed, straight-walled recess that lets a socket-head cap screw or a washer sit below or flush with the surface. Use a countersink for tapered flat-head fasteners, typically 82 degrees in inch and 90 degrees in metric, and a counterbore for cylindrical socket-head screws. A spotface is a shallow counterbore that just cleans up a flat seat for a washer or bolt head.
Countersink
A countersink is an angled conical recess matched to the head angle of a flat-head screw, so the head finishes flush with, or just below, the surface. The common included angles are 82 degrees for inch flat-head screws, 90 degrees for metric, and 100 degrees for some aerospace fasteners. The drawing calls out the diameter at the surface and the angle.
Counterbore
A counterbore is a flat-bottomed cylindrical recess, larger than the clearance hole, that lets a socket-head cap screw or a washer drop below the surface. The callout gives the counterbore diameter and depth, plus the clearance hole beneath it.
Spotface
A spotface is a very shallow counterbore whose only job is to machine a flat, square seat for a bolt head, nut or washer on a rough or curved surface. It removes just enough material to let the fastener seat squarely.
Which to use
| Need | Use |
|---|---|
| Flat-head (tapered) screw sitting flush | Countersink |
| Socket-head cap screw recessed or flush | Counterbore |
| Flat seat for a bolt head or washer on a rough surface | Spotface |
| Pan or button head sitting proud | Clearance hole only |
On the drawing
Modern drawings use symbols. The countersink symbol is a V that opens upward, and the counterbore and spotface share a symbol that looks like a square-bottomed U. Each is followed by its diameter, and a counterbore also gets a depth. Good practice is to name the fastener it is for, so the shop can confirm the geometry. See GD&T explained for how these fit into a fully dimensioned drawing, and the drill and tap size chart for the clearance and tapped holes beneath them.
What to send for a quote
Your drawing with the hole callouts, the fasteners you intend to use, and the material and quantity. If you are early in design, our design for manufacturing guide covers choosing these features well. Request quotes from Canadian CNC machining shops.
FAQ
What is the difference between a counterbore and a countersink?
A countersink is a cone-shaped recess for a tapered flat-head screw to sit flush. A counterbore is a flat-bottomed cylindrical recess for a socket-head screw or a washer to sit below the surface.
What angle is a countersink?
82 degrees for inch flat-head screws, 90 degrees for metric, and 100 degrees for some aerospace fasteners. Match the recess angle to the screw head.
What is a spotface?
A shallow counterbore that machines a flat, square seat for a bolt head, nut or washer on a rough or curved surface.
Which fastener needs a counterbore?
Socket-head cap screws, and anywhere you want a washer or a bolt head recessed below the surface.
How do you call out a counterbore on a drawing?
With the counterbore symbol followed by the counterbore diameter and depth, above the clearance hole size.
Last reviewed: October 2026. Related reading: drill and tap size chart and GD&T explained.
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