Counterbore vs Countersink vs Spotface: Designing Bolt-Head Recesses

A bolt or screw rarely just sits on a flat surface — its head usually needs a machined recess so it seats correctly, sits flush, or has a clean flat to bear against. The three features that do this job are the counterbore, the countersink and the spotface. They look similar on a drawing but solve different problems, and choosing the wrong one means a fastener that protrudes, won’t seat square, or loosens. This guide explains what each does, when to use it, and how to call it out.

Counterbore

A counterbore is a flat-bottomed cylindrical recess machined around a hole. Its job is to let a socket-head cap screw (or a washer) sit below or flush with the surface, with the head bearing on the flat bottom of the bore. Use a counterbore whenever you want a cap screw recessed into the part — for a clean surface, clearance for moving parts, or to protect the fastener. The counterbore diameter clears the screw head, and its depth sets how far the head sits below the surface.

Countersink

A countersink is a conical recess, cut at an angle (commonly 82° in inch standards, 90° in metric) to match a flat-head (countersunk) screw so the head sits perfectly flush with the surface. Use a countersink wherever you need a fastener head dead flush — sheet metal, panels, sliding surfaces, or anywhere a protruding head is unacceptable. The key is matching the countersink angle to the screw head angle, and the diameter so the head finishes flush.

Spotface

A spotface is a shallow, flat machined area around a hole — essentially a very shallow counterbore. Its purpose is to give a fastener head, washer or nut a clean, flat, perpendicular surface to seat on, especially on a rough, curved, cast or angled face. Use a spotface on castings, forgings, weldments and inclined surfaces where the fastener would otherwise bear on an uneven surface and load unevenly. It removes just enough material to create a true seating face.

At a Glance

Designing and Specifying Them

• Match the feature to the fastener. Socket-head cap screw → counterbore; flat-head screw → countersink; bolt/nut on a rough or curved face → spotface.
• Give the counterbore enough depth to recess the head as intended — and enough diameter to clear the head and any tool radius.
• Match the countersink angle to the screw (don’t mix 82° and 90°), or the head won’t sit flush.
• Use standard tooling sizes so the shop isn’t buying special cutters — relates to the cost rules in our cost-reduction guide.
• Add a lead-in or break the edges where helpful, and keep the recess concentric with the hole — see chamfers & fillets and the bolt clearances in our fits & tolerances guide.

The Bottom Line

Counterbore, countersink and spotface each prepare a hole to seat a fastener properly — a flat-bottomed recess for a cap screw, a matching cone for a flush flat-head, and a shallow flat to true up a rough or curved face. Pick the one that suits the fastener and the surface, size it to standard tooling, and call out the angle and depth clearly. These small features are the difference between a fastener that seats square and holds, and one that protrudes, cocks or works loose.

MechPart Pro machines counterbores, countersinks and spotfaces to your drawing, and our engineers will confirm the right recess, angle and tooling for your fasteners as part of our free design-for-manufacturability review. Pair it with the right material from our materials guide and reference data in our engineering charts.