Buy Counterbores Online in Australia
Counterbore Selection — Quick Reference
Counterbores produce FLAT-BOTTOMED RECESSED HOLES allowing SOCKET HEAD CAP SCREWS + similar fasteners to sit FLUSH with or BELOW the surface. Drill press + lathe + machining centre + workshop standard.
| Counterbore Type | Best For |
|---|---|
| Standard Counterbore (Pilot) | SHCS recess — common workshop |
| Interchangeable Pilot | Multiple SHCS sizes from one counterbore |
| Carbide-Tipped Counterbore | Hardened material + production |
| HSS Standard | Mild + low-alloy steel |
| Spotface Counterbore | Flat seating surface — no recess |
| Set (Workshop Range) | SHCS M4-M20 typical |
Critical: Pilot diameter MATCHES screw clearance hole (M8 SHCS = 8.5mm pilot + 14mm body counterbore for socket head). SLOW SPEED + cutting fluid. Brands: Sutton Tools, Bordo. Companion: countersink bit sets, socket head cap screws, drilling.
Counterbores
Counterbores produce flat-bottomed recessed holes that allow socket head cap screws and similar fasteners to sit flush with or below the surface of the workpiece — essential for assemblies where a projecting fastener head would interfere with mating components, guards or covers. AIMS Industrial stocks HSS counterbores in standard pilot sizes for M3 to M20 socket head cap screws for use in drill presses and milling machines.
Counterbore vs Countersink
A counterbore produces a flat-bottomed recess with straight sides — used for socket head cap screws (Allen cap screws) and flanged bolts whose heads are cylindrical. A countersink produces an angled (typically 90° or 82°) tapered recess — used for countersunk head (flat head) screws where the tapered head must sit flush. The application determines which is needed: for any fastener with a cylindrical or flanged head that must be recessed below the surface, a counterbore is the correct tool.
Pilot Pin and Size Selection
Counterbores are sized by the outer diameter (matching the cap screw head diameter plus clearance) and the pilot pin diameter (matching the bolt clearance hole diameter). The pilot pin ensures the counterbore is concentric with the pre-drilled clearance hole. Standard counterbore sets cover the common cap screw sizes — M3, M4, M5, M6, M8, M10, M12, M16 and M20 — in pilot diameters to suit standard or close-tolerance clearance holes.
Material and Use
HSS counterbores suit mild steel, cast iron and aluminium for general engineering and maintenance applications. For harder materials or high-volume CNC use, carbide-tipped or solid carbide versions are available. For help selecting the right counterbore size for your fastener and material, contact our team. AIMS Industrial has been supporting Australian workshops since 1988.
People Also Ask — Counterbores
Q: How deep should a counterbore be cut for a socket head cap screw to sit flush?
Counterbore depth should equal the fastener head height plus a small allowance, typically an extra 0.5-1mm, to guarantee the head sits fully below or flush with the surface even accounting for machining tolerance. Cut too shallow and the head stays proud; cut too deep and you weaken the surrounding material, especially in thinner sections. Head height varies by screw diameter and standard (ISO 4762 metric versus ASME B18.3 imperial), so check the relevant spec rather than assuming a standard depth.
Q: Do I need a different counterbore size for a hex bolt head than a socket head cap screw?
Yes — a hex bolt head is wider across the flats than a socket head cap screw of the same thread size, so it needs a larger-diameter counterbore body even though the pilot hole, matched to the thread's clearance hole, stays the same. Flanged hex bolts need a wider body again to clear the flange. Always counterbore to the actual head or flange diameter of the fastener you're using, not a generic size-for-size assumption based on thread diameter alone.
Q: Should I use a carbide-tipped or HSS counterbore?
HSS (high speed steel) counterbores handle mild and low-alloy steel in standard workshop use and are the more affordable option. Carbide-tipped counterbores hold their edge in hardened material and high-volume production work where HSS would wear out faster — worth the extra cost when cutting hardened steel or running high job volumes.
Q: What's a spotface counterbore used for?
A spotface counterbore cuts a shallow flat seating surface rather than a full recess — used where a bolt or washer needs a clean, flat bearing surface on a rough or curved casting, but the fastener head doesn't need to sit below the surface. Run counterbores at slow speed with cutting fluid to control the finish and tool life.