Quick answer: UNJ (inch) and MJ (metric) are aerospace thread forms with a controlled, rounded root radius on the external thread, which greatly improves fatigue strength. They must be ordered as UNJ or MJ gauges — standard UN or M gauges do not reflect the J-form minor diameter, and a ring gauge alone cannot prove the root radius is correct.
Need gauges for this? GageTools manufactures GO/NO-GO thread plug gauges and thread ring gauges in Dongguan, China. Standard sizes ship from stock within 72 hours, MOQ is 1 piece, and every RFQ gets a reply within 24 hours — request a quote.
UN vs UNJ, M vs MJ: What Is Different
| Feature | Standard UN / M | UNJ / MJ |
|---|---|---|
| External root | Flat or optional round | Mandatory controlled radius |
| Internal minor diameter | Standard | Increased to clear the radius |
| Typical classes | 2A/2B, 6g/6H | 3A/3B (UNJ), 4h6h/4H5H (MJ) |
| Standards | ASME B1.1, ISO 965 | ASME B1.15 (formerly MIL-S-8879), ISO 5855 |
| Typical use | General industry | Aircraft structures, engines, fasteners |
Which Gauges an Aerospace Thread Needs
- Internal UNJ/MJ threads: GO and NO-GO thread plug gauges made to the J-form limits, plus a plain plug gauge if the drawing controls the increased minor diameter.
- External UNJ/MJ threads: GO and NO-GO thread ring gauges with matching setting plugs. The root radius is verified separately, usually by optical comparator or profile projector, because a ring gauge does not contact the root.
- Documentation: most aerospace customers need a calibration certificate with measured values and traceability. ISO/IEC 17025 certificates are available on request.
Coatings, Lubricants and Blind Holes
Aerospace fasteners are often cadmium-, silver- or dry-film coated. Tell your gauge maker whether parts are inspected before or after coating so the gauge limits match. For blind holes, send the full thread depth and any undercut details so the plug gauge length and the NO-GO member are made correctly.
Common Aerospace Gauging Pitfalls
- Accepting UNJ parts with standard UN gauges because the pitch diameter “looks the same”.
- Skipping root-radius inspection because the ring gauge passed.
- Ordering gauges without stating the class (3A/3B or 4h6h/4H5H).
- Not requesting measured calibration data that the customer’s AS9100 system requires.
Frequently Asked Questions
Can I check a UNJ internal thread with a UN 3B plug gauge?
Not completely. The J-form minor diameter differs, so use gauges specified for UNJ and follow the drawing’s standard.
What does the J in UNJ mean?
It marks the controlled root radius thread form defined for aerospace use, originally in MIL-S-8879 and now in ASME B1.15.
Do you make MJ gauges in large diameters?
Send the callout and quantity; custom sizes are made from drawing, typically 10–15 working days after approval.
Related Buyer Guides
- Thread Gauges for Aerospace: AS9100, NIST Traceability and Special Requirements
- Thread Class 2B vs 3B: Which Thread Gauge Do You Need?
- What Is NIST Traceability for Thread Gauges?
- Thread Gauges Before or After Plating
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Request UNJ or MJ Aerospace Thread Gauges
Send your thread callout, tolerance class and quantity — or just your drawing. Standard sizes ship from stock within 72 hours, custom gauges in 10–15 working days, MOQ 1 piece. Reach us on WhatsApp at +86 173 1689 5054 or [email protected].