UNC & UNF Tap Drill Chart: Imperial Threads with Metric Drill Equivalents

Metric may run the drawing office, but the workshop floor from Karachi to Manila still bolts together a great deal of UNC and UNF — American-built machinery, generator sets, truck fleets, pumps, and every piece of equipment that arrived second-hand from a US-standard market. Those threads need re-tapping like any others, and the tap drill chart for them is generated by the same kind of one-line rule as the metric chart:

Tap drill = major diameter − (25.4 ÷ TPI) millimetres

A ¼-20 thread: 6.35 − (25.4 ÷ 20) = 6.35 − 1.27 = 5.08 mm — the #7 drill, or a 5.1 mm metric bit from the index you already own. That is the whole trick, and it lands the same ~75% thread engagement the metric rule does. The charts below just save the arithmetic.

UNC (coarse) tap drill chart

UNF (fine) tap drill chart

The metric column is not a compromise — at these sizes the nearest 0.1 mm bit sits within the normal tap drill tolerance. Where the metric bit runs a shade small (4.0 for #10-32, 5.4 for ¼-28), it simply cuts a slightly fuller thread; in stainless or spring steel, go up a tenth instead — the why is in the 75% engagement section of the metric guide, and the number and letter drill systems themselves are decoded in the drill size chart.

NPT pipe taps: the tapered exception

NPT threads seal on their taper, so the tap only enters until the taper binds — thread it in the workpiece deep enough that a fitting engages 4–5 turns hand-tight. External pipe threads run the same sizes from the die side, which is exactly what an adjustable NPT threading die is for.

The third system: Whitworth, on British-era machines

Workshops in India and Pakistan meet one more standard the charts above do not cover: BSW (Whitworth), on machinery from the British era and its descendants. It looks close enough to UNC to cause real damage — several sizes share the same TPI — but the flank angle is 55° against Unified’s 60°, so mixed bolts start by hand and strip under load. The tell-tale size: a ½" Whitworth runs 12 TPI where ½-13 UNC runs 13. Same drill rule, though:

From our range

Industrial HSS Spiral Point Machine Tap (Through Hole Dedicated) – OEM

Spiral point taps for through holes — chips exit forward, torque stays low. Available in UNC/UNF on OEM order.

From our range

3/8"-18 Industrial HSS Adjustable Round Threading Die (NPT Pipe Thread) – OEM

Adjustable round die for NPT pipe threads — the repair-bench standard for sealing threads.

Everything else transfers from metric

• Chip strategy is identical: spiral point for through holes, spiral flute for blind holes, straight flute for hand work and cast iron.
• Blind holes: drill 4 × pitch deeper than the thread you need — with TPI, pitch is 25.4/TPI mm, so ¼-20 wants ~5 mm of spare hole.
• Speed: tap at a fraction of drilling speed with cutting compound; in stainless, halve it again and never stop mid-thread.
• Mixed benches: label the drawer. A ¼-20 bolt starts in an M6 nut and strips it in three turns — the two systems do not interchange, only cohabit.
• Dies mirror the chart: the blank for an external thread is the major diameter itself — bar stock at 6.35 mm for ¼-20, and the die cuts the rest.

For the counter and the container

Any market that imports second-hand plant needs imperial taps beside the metric rack — ¼-20, ⁵⁄₁₆-18 and ⅜-16 UNC cover most repair work, with NPT dies close behind wherever there is pipework. We supply HSS machine taps in unified and NPT sizes as OEM production runs, ground thread, in the same container as the metric range — one shipment, both standards on the shelf.