UNF Thread Gauge Size Chart: Sizes, TPI, Tap Drills and Uses

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UNF Size Chart

Tap drills listed give roughly 75% thread engagement.

Where UNF Threads Are Used

UNF vs UNC vs UNEF

UNF Threads in Hydraulic Fittings

SAE hydraulic fittings use UNF threads identified by dash size. The same thread appears on O-ring boss (ORB) ports and on JIC 37° flare fittings of the same dash size, which is convenient for gauging:

The 1 1/16-12 thread on -12 fittings is from the UN 12-pitch series, not standard UNF, but it is gauged the same way with 2B plug and 2A ring gauges.

Nearest Metric Equivalents

Class 3A/3B and UNJF for Aerospace

Aerospace drawings often specify UNF threads in class 3A/3B, or UNJF threads with a controlled root radius. Class 3 gauges have tighter limits and no allowance on the external thread. UNJF external threads cannot be fully verified by a ring gauge alone; the root radius is checked separately, usually optically.

Why Designers Choose UNF

UNF gives more threads per inch than UNC for the same diameter. That means a smaller lead angle, which resists self-loosening under vibration; a larger tensile stress area, which makes the bolt slightly stronger; and a shallower thread, which suits thin walls and tubes.

The trade-offs are the same as metric fine threads: more turns to assemble, more risk of cross-threading and less tolerance for dirt and damage.

Gauging UNF Threads in Production

• Deburr threads before gauging — fine flanks are easily affected by burrs.
• Start gauges square to avoid cross-threading.
• Gauge blind hydraulic ports to full depth.
• Keep UNF and metric fine gauges in separate, labelled locations, especially 3/4-16 and M18x1.5.
• Replace GO members on schedule; fine threads wear gauges noticeably in high-volume use.

Small UNF Sizes

#10-32 is especially common — on pneumatic fittings, optical mounts and instruments. It is one of the most-ordered inch fine gauges.

When to Use UNEF Instead

UNEF (extra fine) threads go one step further than UNF. They are used where the wall is very thin or where many threads are needed in a short length — for example, on tubing, bearing locknuts, and some aerospace and instrument parts.

UNEF gauges are made to order in most sizes. Send the full callout, e.g. 1/2-28 UNEF-2B.

Wear on High-Volume UNF Lines

Hydraulic fitting production can run thousands of parts per shift on the same size — often 7/16-20, 9/16-18 and 3/4-16. GO members on these lines wear quickly. Monitor pitch diameter at every calibration, shorten intervals as the gauge nears its wear limit, and keep calibrated spare GO members so production never stops for a gauge.

Length of Engagement for Fine Threads

Fine threads have more threads per inch, so the same engagement length contains more threads. That is useful in thin walls, but it also means small pitch (lead) errors accumulate more over the length. The GO gauge, which is full-form and full-length, catches this accumulated error. If a fine-thread part passes the NO-GO gauge but fails the GO gauge, lead error is a likely cause worth investigating at the tapping or thread-rolling stage.

Sourcing UNF Gauges Internationally

Many buyers outside North America need UNF gauges for exported equipment or imported machines. When sourcing:

• Specify the full ASME designation, e.g. 9/16-18 UNF-2B, not just “9/16 fine”.
• State the gauge standard (ASME B1.2) and whether ring gauges should be adjustable with setting plugs.
• Ask for certificates with measured values if your customer requires them.
• Check lead times for less common sizes.

Summary

UNF threads are the standard fine inch series, essential for SAE hydraulic fittings and common in aerospace and precision work. Each size and class needs its own GO/NO-GO gauges, UNJF threads need J-form gauges, and look-alike metric sizes such as M18x1.5 must be kept firmly separate. With the right gauges and a clear inspection plan, UNF threads are easy to control in production.

Key Terms

Quick definitions of the terms used in this guide:

Practical Tips for UNF Inspection

Fine inch threads reward careful handling. These habits help:

• Start gauges square — cross-threading damages both gauge and part.
• Blow out blind ports before gauging; chips hide easily in fine threads.
• Label 3/4-16 UNF and M18x1.5 gauges in contrasting colours.
• Check GO member wear more often on high-volume hydraulic sizes.
• Record rejects by size to spot tapping problems early.

Next Steps

To order UNF gauges, list each size with its class, and say whether any threads are UNJF. For hydraulic work, you can simply send the dash sizes and fitting standard (ORB or JIC) and we will translate them into the correct thread gauges. Include setting plugs for ring gauges if you calibrate in-house, and any certificate requirement. We reply with price and lead time within 24 hours, and common sizes are often in stock for dispatch within 72 hours.

How to Order UNF Thread Gauges

• Full callout, e.g. 7/16-20 UNF-2B
• Plug and/or ring, quantity
• Setting plugs for ring gauges if needed
• Certificate requirement

Frequently Asked Questions