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A clearance hole lets a machine screw pass freely through one part while its threads engage in a second part, or a nut on the far side. The hole must be large enough that the screw slides through without binding, but not so large that the head, washer, or mating part loses support.
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Choose the hole size
Start with the screw’s nominal diameter and the fit you need. A close-fit hole keeps the parts aligned; a free-fit hole gives more room for imperfect alignment. For ordinary workshop assembly, a free-fit clearance hole is usually the practical choice.
| Screw size | Close-fit drill | Free-fit drill |
|---|---|---|
| #6 (0.138 in) | 9/64 in (0.141 in) | 5/32 in (0.156 in) |
| #8 (0.164 in) | 11/64 in (0.172 in) | 3/16 in (0.188 in) |
| #10 (0.190 in) | 13/64 in (0.203 in) | 7/32 in (0.219 in) |
| 1/4 in | 17/64 in (0.266 in) | 9/32 in (0.281 in) |
| 5/16 in | 21/64 in (0.328 in) | 11/32 in (0.344 in) |
These are common inch-series sizes, not universal rules. Check the screw manufacturer’s chart when the fit is critical, or when working with metric screws, countersunk heads, or unusually thick parts. A fractional drill bit set covers many common inch sizes; a numbered or metric set may be a better match for the hardware you use most.
Pick the bit and drill
Use a bit intended for the material: a standard high-speed-steel twist bit works for many wood and mild-steel jobs, while masonry needs a masonry bit. For metal, a sharp bit cuts cleaner and needs less pressure than a dull one. If you are buying bits mainly for metalwork, a cobalt bit set can be useful for tougher alloys, but it is unnecessary for occasional holes in soft steel or wood.
A cordless drill is the right tool for most clearance holes. Use a hammer drill only in hammer mode for masonry; turn hammer action off for wood and metal, where it can damage the bit and workpiece. An impact driver can turn a hex-shank drill bit, but its hammering action makes it harder to control and can break small bits. A regular drill gives better control for accurately sized holes.
Mark and secure the work
Measure and mark the hole center. On metal, a center punch makes a small dimple that helps prevent the bit from wandering; a nail set or sharp punch can work for light-duty jobs. On wood, a brad-point bit or a small pilot hole helps locate the larger bit.
Clamp the work securely to a bench or sacrificial backing board. Do not hold a small metal piece in your hand while drilling: if the bit catches, the piece can spin. Check behind the work for wires, pipes, or other obstructions before drilling into anything installed.
Drill the clearance hole
Fit the selected bit straight in the chuck and tighten it securely. Mark the required depth if the hole is blind, or place scrap wood behind the work to limit tear-out as the bit exits. For a through-hole, keep the drill square to the surface; a drill guide or square held beside the bit can help if alignment matters.
Start at a steady, moderate speed with light pressure. Let the cutting edges do the work. For a large hole in metal, drilling a smaller pilot hole first can reduce wandering, but it is not required for every job. Back the bit out occasionally to clear chips, especially in deeper holes. Use cutting fluid on steel if appropriate, and clear chips with a brush—not your fingers.
Near the far side of wood, ease off the pressure to reduce splintering. In thin sheet metal, support the back with scrap wood and slow down as the bit breaks through; thin stock can grab abruptly. Wear eye protection, and avoid loose gloves, sleeves, or hair near rotating equipment.
Check the fit and fix problems
After drilling, remove burrs with a deburring tool or a light pass with a larger bit turned by hand. Test the screw through the hole before final assembly. It should pass without forcing, while the head or washer still covers the opening with enough bearing surface.
If the screw binds, the hole may be undersized, off-center, or angled. Do not force the screw through: it can damage the threads or pull the parts out of alignment. Enlarge the hole carefully with the next bit size, or use a round file if only a small correction is needed. If the hole is too large, a larger washer may restore support; if alignment or strength is important, remake the part rather than relying on a loose fit.
For a hole that needs a screw head to sit flush, a clearance hole alone is not enough. Add a matching countersink for a flat-head screw, or a counterbore for a socket-head screw, and keep the recess centered over the clearance hole. Test the head fit on scrap first: too deep a recess can leave too little material under the head.