Counterbore & Clearance Hole Standards Explained

Clearance Hole Sizes: Close, Normal, and Loose Fit

A clearance hole is simply a hole that is larger than the bolt body diameter, allowing the bolt to pass through without threading. The size of the clearance determines the fit class. ASME B18.2 and ISO 273 define three fit classes that cover the full range of assembly needs.

Close Fit provides the minimum practical clearance — typically 1/64 to 1/32 inch larger than the bolt body diameter for inch sizes. Use close fit when the bolt pattern must align precisely, as in dowel-located fixtures or precision mating surfaces where positional accuracy matters more than ease of assembly.

Normal Fit is the default for general manufacturing. It provides enough clearance to accommodate standard machining tolerances on hole location and bolt diameter variation. Normal fit allows the bolt to slide through easily without excessive slop. This is the right choice for 80% of machine shop applications.

Loose Fit adds extra clearance for field assembly, alignment adjustment, and slotted holes. Use it when parts may not align perfectly due to fabrication tolerances, thermal expansion, or the need for positional adjustment during assembly.

Counterbore Depth: How Deep Is Deep Enough

The minimum counterbore depth equals the screw head height. A socket head cap screw (SHCS) must sit with its head at or below the surface of the part for most applications. But minimum is not the same as recommended — machining a counterbore to exactly head height leaves zero margin for surface irregularities, burrs, and screw head tolerance.

For CNC machined counterbores, add 0.010 to 0.020 inches beyond the head height. This extra depth ensures the head always clears the surface even with a slightly oversize head (within tolerance) or a slight burr on the counterbore bottom. Program the counterbore depth as head height + 0.015 inch and you will never have fitment problems.

For counterbores machined on manual equipment (bridgeport, drill press), add 0.020 to 0.030 inches. Manual depth control is less precise than CNC, and the extra margin accommodates depth variation across a bolt pattern. It is always better to have the head slightly below flush than slightly above.

The counterbore diameter should be the head diameter plus 1/32 to 1/16 inch clearance. This allows the screw to enter the counterbore easily without binding on the walls. A tight counterbore can cock the screw and prevent it from seating flat, which reduces clamping force and can cause the screw to fatigue under cyclic loading.

Common Mistakes and How to Avoid Them

Undersized clearance holes: The most common mistake is drilling the clearance hole too small, either by using the bolt nominal diameter instead of the clearance size, or by using a worn drill that cuts undersize. The bolt jams in the hole, the assembler forces it through with a hammer, and the threads are damaged. Always use a clearance hole chart, and check drills with a micrometer if they have been resharpened.

Insufficient counterbore depth: A socket head sitting 0.005 inches above the surface creates a stress concentration on the mating part and prevents the joint from developing full clamping force. It also means the next part in the stack does not sit flat. When in doubt, go deeper. A screw head sitting 0.020 below flush causes no problems; a screw head sitting 0.005 above flush causes many.

Counterbore not perpendicular to the surface: On angled or curved surfaces, the counterbore must be perpendicular to the bolt axis, not the local surface. A cocked counterbore causes the screw head to bear on one edge, concentrating stress and reducing clamping force. On manual machines, use a piloted counterbore that follows the clearance hole to maintain alignment.

Mixing inch and metric: An M6 SHCS head diameter is 10mm (0.394"). A 1/4" SHCS head diameter is 0.375". The counterbore for M6 is larger than for 1/4" even though the bolt bodies are nearly the same size. Always verify the screw system (inch vs metric) before looking up counterbore dimensions.

Countersinks for Flat Head Screws

Flat head (countersunk) screws sit flush with the surface. The countersink angle must match the screw head angle exactly: 82 degrees included for inch screws (per ASME B18.3) and 90 degrees included for metric screws (per ISO 10642). Using the wrong angle creates either a ring contact (screw sits high) or a point contact (screw sits too deep and wobbles).

Countersink the hole to a diameter that allows the screw head to sit flush or very slightly below the surface. The finished countersink diameter should equal the screw head diameter plus 0.005 to 0.010 inches. Going deeper makes the head sit below flush, which is acceptable for most applications but looks sloppy. Going too shallow leaves the head proud, which interferes with mating parts.

For CNC countersinking, use a single-flute or multi-flute countersink tool and program to a depth that produces the correct diameter. The relationship between depth and diameter is fixed by the angle: for an 82-degree countersink, every 0.001 inch of additional depth increases the diameter by approximately 0.0035 inches.