Engineering · Tool 02
Bolt Clearance & Tap Drill Sizes
Clearance Hole · Tap Drill · Pitch
Three related lookups, kept separate on purpose: the clearance hole a bolt shank passes through, the tap drill that's threaded to receive it, and which fit class to reach for and when. Same nominal bolt size, three different answers depending on what the hole is actually for.
Reference only. Clearance sizes follow the ISO 273 standard series; tap drill sizes use the standard ~75% thread engagement rule of thumb for coarse-pitch metric threads. Confirm against a drill/tap chart or the fastener spec for anything precision-fit or load-critical.
Bolt Size vs. Drill Size
The clearance hole a bolt's shank passes through — sized so the bolt slides in freely and the second part or a nut does the threading.
| Bolt size | Close fit | Medium fit | Free fit |
|---|
Tap Size vs. Drill Size
The tap drill — sized smaller than the bolt, leaving material for the tap to cut internal threads into.
| Thread size | Pitch | Tap drill | Minor Ø (theoretical) |
|---|
Fit Class vs. Applications
Which clearance fit to reach for, by what the joint actually needs to do.
| Fit class | Clearance | Typical applications |
|---|---|---|
| Close fit | Minimal — bolt nearly fills the hole | Precision location where parts must stay aligned: jigs and fixtures, dowel-adjacent joints, indexing plates, bushings, anywhere lateral shift can't be tolerated. |
| Medium fit | Moderate — general working clearance | The general-purpose default. Most bolted machine joints, structural brackets, everyday fastening where some play is fine and alignment isn't critical. |
| Free fit | Generous — easy running clearance | Quick assembly and disassembly, slotted or adjustable joints, field assembly where hole alignment between parts is imprecise, access covers and panels. |
Explanatory Notes
Clearance hole vs. tap drill
A clearance hole is drilled larger than the bolt so the shank passes through freely. A tap drill hole is drilled smaller than the bolt's nominal diameter, leaving material for the tap to cut internal threads into. Same nominal size, two very different drills — easy to mix up, and it's usually only caught after the hole's already drilled.
Why ~75% thread engagement
The tap drill sizes here target roughly 75% thread engagement — the widely used default for general steel and aluminium tapping. Full 100% engagement barely adds strength over 75% but makes tapping noticeably harder and more likely to break a tap, especially by hand. Softer or thin-walled materials sometimes call for higher engagement to make up for lower material strength; harder materials often go lower, around 50–65%, specifically to ease tapping.
Blind vs. through holes
In a through hole, chips clear out the far side and full thread length is straightforward. In a blind hole, chips pack at the bottom, and the tap's own chamfered lead (typically 3–5 threads) can't cut full-depth threads — so the last few threads at the bottom are always incomplete. Always drill a blind hole deeper than the thread length you actually need.
When the table isn't enough
These are general machine-shop defaults. For structural connections, pressure-boundary joints, or anything with a specified fastener class, use the governing standard or fastener manufacturer's chart directly rather than this general reference.
Basis: clearance hole diameters per the ISO 273 clearance-hole series (close/medium/free fit classes); tap drill diameter approximated as nominal diameter minus pitch (Tap Drill ≈ D − P), the standard rule of thumb for ~75% engagement on coarse-pitch metric threads; theoretical minor diameter shown for reference only, not as a drill recommendation.