Metric, Inch and Pipe Thread Systems: How They Differ

What a Thread Callout Encodes

Two callouts on the same drawing can both describe a thread with a 60-degree included angle and still send the shop to different gages, different tables and different acceptance rules. Which system a callout belongs to is fixed by the callout itself, not by the part it sits on.

A callout carries four elements: the thread form or profile, the nominal size, the pitch, and the tolerance class. FED-STD-H28/20B (1994) §5.5 adds a fifth when acceptance matters: the standard gaging system number goes in parentheses after the tolerance class designation, and an “S” is appended to that number when the gaging system is a modification of a standard one. The same section also fixes where the inspection method is recorded — in a general note in the procurement package, at the individual thread callout, or as a combination of the two.

The numbers behind a single callout are worth seeing in full. FED-STD-H28/20B (1994) §5.7.1 works through 1/4-28 UNF-3A: the pitch diameter product tolerance is 0.0025 inch, so the smallest scale division on the indicating gage dial is set at one tenth of that tolerance, 0.00025 inch. Nothing in the callout itself tells a shop which of those numbers to look up; the system, and the standard that governs it, do.

Metric M and MJ: 60-Degree Profile in Millimeters

FED-STD-H28/21B (1984) §1 defines its scope as general standards for 60-degree symmetrical threads, and it covers two profiles, M and MJ. §1.1 of the same section sets the size floor at 1.6 mm and larger, and assigns smaller metric sizes to a different section of the standard.

The M profile is the general-purpose form. FED-STD-H28/21B (1984) §5.1.1 fixes its standard tolerance classes at 6g for external threads and 6H for internal threads. The MJ profile covers a different duty. FED-STD-H28/21B (1984) §5.2.1 gives its standard external class as 4h6h, and its internal class as 4H6H for sizes 1.6 mm through 5 mm and 4H5H for 5.5 mm and larger. The note to §1.2.2 records the geometric reason the profiles are kept apart: the MJ external thread has a mandatory controlled rounded root and an increased minor diameter, and the note states that rolled threads with rounded roots more than double part fatigue life.

Two further limits are numeric. FED-STD-H28/21B (1984) §4.4.2 caps thread surface texture at 2.5 µm Ra for cut threads and 1.6 µm for rolled and ground threads. FED-STD-H28/21B (1984) §4.6.2.1 limits the lead-in thread to 2P, measured from the end of the product to the start of the first complete thread, and §4.6.1 defines a completely formed thread as one that follows the profile within the tolerance zone over an axial distance of one pitch.

Reading the designation: FED-STD-H28/21B (1984) §5.1.4 places M profile designation in section 7 of ANSI/ASME B1.13M-1983, and §5.2.4 places MJ profile designation in section 7 of ANSI B1.21M-1978. The worked examples in the section are written as nominal size, then pitch, then tolerance class — M27x1.5-6H in §5.1.3 and MJS 6.5x1-4h6h in §5.2.3. For tapping holes derived from a metric callout, the site’s tap drill chart is organised by size and pitch.

Unified Inch Threads: UN, UNR and UNJ

FED-STD-H28/20B (1994) is an inspection section, and its Foreword states that it covers products using 60-degree symmetrical inch and metric screw threads. §1.1 sets the family list for that scope: UN, UNR, UNJ, M and MJ, with a further note that the section may also be applied to Class 5 interference fit threads.

The tolerance structure in this family is expressed through gaging systems rather than through a single class table. FED-STD-H28/20B (1994) §5.1.1 establishes three of them, identified as Systems 21, 22 and 23, and §5.1.1.1 places all three in accordance with ASME B1.3M-1992, with a note reserving System 21A to metric threads. What each system controls is stated separately. FED-STD-H28/20B (1994) §5.1.3.1 has System 21 control functional size at the maximum material limits within the length of the standard gaging elements, plus NOT-GO functional diameters and HI (internal) and LO (external) functional diameters. FED-STD-H28/20B (1994) §5.1.4.1 has System 22 control functional size at the maximum material limits within the length of the standard gaging elements and minimum material size limits over the length of the full thread, and, for UNJ and MJ external threads, the thread root radius and the rounded root minor diameter as well. Selection follows the features that matter to the end use, and one of the stated conditions is a size threshold: FED-STD-H28/20B (1994) §5.1.3.2(d) lists an internal thread smaller than 0.190 inch (5 mm) nominal size among the conditions under which System 21 is suggested.

The section also fixes instrument and gage numbers a shop has to meet. FED-STD-H28/20B (1994) §5.7.1 requires the smallest scale division on an indicating gage to be no greater than ten percent of the product feature tolerance, and the accuracy of the indicating system to be within ten percent of that tolerance. FED-STD-H28/20B (1994) §5.7.2 sets setting-gage calibration accuracy at the lesser of either half the gage tolerance or 0.0002 inch (0.005 mm), and requires the dimensional standards used to reference measuring instruments to be accurate to 10 microinches (0.00025 mm). FED-STD-H28/20B (1994) §5.8.3 caps gaging-surface roughness at 16 microinches (0.4 µm) Ra for thread gage crests and 8 microinches (0.2 µm) for thread flanks and plain gaging contacts. For optical projection, FED-STD-H28/20B (1994) §5.6.3 sets minimum magnification at 20X for threads up to 14 per inch, 50X from 14 up to 40, and 100X for 40 and above.

Reading the designation: the section’s own example, 1/4-28 UNF-3A in FED-STD-H28/20B (1994) §5.7.1, carries a fraction size, a thread count per inch, the series letters and the tolerance class. For inch fastener thread tables, the site’s UNC/UNF thread chart is the entry point.

Pipe Threads: A Separate Inch Series

Pipe threads are governed separately from fastener threads. FED-STD-H28/7A (1984) §1 covers general-purpose pipe threads only, and §1.1 states what falls outside it: dryseal pipe threads (NPTF, PTF-SAE SHORT, NPSF, NPSI), hose coupling and fire-hose coupling threads (NH, NPSH), and aeronautical national form taper pipe threads (ANPT).

The design content of this system is delegated rather than tabulated in the section. FED-STD-H28/7A (1984) §4 requires thread designs, designations and inspection requirements to follow ANSI/ASME B1.20.1-1983, with gages and gaging following that standard and the alternatives in §5. Of those alternatives, FED-STD-H28/7A (1984) §5.2 describes zero-lead triroll gages used to check external thread pitch diameter, taper, lead and flank angles.

One thread form appears with its dimensions in full. FED-STD-H28/7A (1984) Appendix B §50.2 records the special thread form used on drum closures for dangerous-goods containers: thread angle 60 degrees, depth of thread 0.0933 inch, and radii of crest and root 0.0075 inch. The internal thread in the flange is 8 threads per inch with a pitch diameter of 2.2067 inch, a maximum major diameter of 2.305 inch and a minimum major diameter of 2.295 inch. That is an appendix of special pipe thread applications, not the general-purpose series; the same appendix, at FED-STD-H28/7A (1984) Appendix B §50.1, records that standard drum bung and vent flanges are tapped with 2 inch and 3/4 inch straight pipe threads.

Tolerance Classes and Gaging Systems Side by Side

Profile or system	Standard tolerance class	Stated in
Metric M, external	6g	FED-STD-H28/21B (1984) §5.1.1
Metric M, internal	6H	FED-STD-H28/21B (1984) §5.1.1
Metric MJ, external	4h6h	FED-STD-H28/21B (1984) §5.2.1
Metric MJ, internal, 1.6 mm through 5 mm	4H6H	FED-STD-H28/21B (1984) §5.2.1
Metric MJ, internal, 5.5 mm and larger	4H5H	FED-STD-H28/21B (1984) §5.2.1
Metric applications that have used inch class 2A/2B	6H internal / 6g external	FED-STD-H28/21B (1984) §1.2.1
Metric applications that have used inch class 3A/3B and UNJ	MJ profile, 4h6h external	FED-STD-H28/21B (1984) §1.2.2 and §5.2.1
Inch and metric product threads, acceptance	Systems 21, 22, 23	FED-STD-H28/20B (1994) §5.1.1
Inch classes named by the metric section	2A/2B and 3A/3B	FED-STD-H28/21B (1984) §1.2.1 and §1.2.2

The table lists only what the cited sections state about classes. The limits of size that belong to those classes are tabulated in the standards those sections incorporate, not in the sections themselves.

Where the Three Systems Meet

The documents draw the metric-inch relationship in their own words, and they draw it as usage history rather than as a table of exchange. FED-STD-H28/21B (1984) §1.2.1 states that the M profile with standard tolerance class 6H/6g is to be used for metric applications where inch class 2A/2B has been used. FED-STD-H28/21B (1984) §1.2.2 states that the MJ profile is to be used for metric applications where inch class 3A/3B and UNJ threads have been used.

The 60-degree angle is stated for each system inside that system’s own scope. FED-STD-H28/21B (1984) §1 states 60 degrees for the M and MJ profiles. FED-STD-H28/20B (1994) Foreword states a 60-degree symmetrical form for the inch and metric threads that section covers. FED-STD-H28/7A (1984) Appendix B §50.2 records a 60-degree thread angle for a special pipe thread form. Three statements, three scopes, and none of the three is stated in terms of another.

The practical crossing point is the incorporation chain. FED-STD-H28/21B (1984) §4.1 places M profile threads in accordance with ANSI/ASME B1.13M-1983 and MJ profile threads in accordance with ANSI B1.21M-1978. FED-STD-H28/7A (1984) §4 places pipe thread design, designation and inspection in accordance with ANSI/ASME B1.20.1-1983. FED-STD-H28/20B (1994) §5.1.1.1 places its gaging systems in accordance with ASME B1.3M-1992. A callout on its own names only the system; the dimensional tables behind it sit in the incorporated document. Reading callouts from two systems therefore means holding two of those documents, not one.

Callout Reading Matrix

Callout as printed in the source	Family	Elements carried	Tolerance class	Source
M27x1.5-6H	Metric M, internal	nominal size, pitch, class	6H	FED-STD-H28/21B (1984) §5.1.3
MJS 6.5x1-4h6h	Metric MJ, special size	nominal size, pitch, class	4h6h	FED-STD-H28/21B (1984) §5.2.3
1/4-28 UNF-3A	Unified inch, UNF series	fraction size, threads per inch, series letters, class	3A	FED-STD-H28/20B (1994) §5.7.1
2 in and 3/4 in straight pipe threads	Pipe, special drum-closure application	nominal size in inches	delegated by §4 to ANSI/ASME B1.20.1-1983	FED-STD-H28/7A (1984) Appendix B §50.1

Fastener size and pitch data in table form is also collected on the site in the bolt size chart.

What This Entry Does Not Cover

The copies consulted for this entry are FED-STD-H28/21B, FED-STD-H28/20B and FED-STD-H28/7A, in the editions dated 28 November 1984, 10 March 1994 and 25 October 1984. The distributed PDF copies carry a notice asking the reader to verify the current version at the source before use.

Two consequences follow. Limits-of-size tables for metric and inch fastener threads are not reproduced in those three sections, so no limit dimension appears in this entry; the class designations in the tables above are printed as the sections print them, and the limits that belong to them are tabulated in the incorporated standards those sections name. And the values quoted here are the values printed in the editions named, so a drawing that cites the same designation but a different edition inherits whatever changed between them.

References

• FED-STD-H28/20B, issued 10 March 1994 — Section 20: Inspection Methods for Acceptability of UN, UNR, UNJ, M and MJ Screw-Threads
• FED-STD-H28/21B, issued 28 November 1984 — Section 21: Metric Screw-Threads
• FED-STD-H28/7A, issued 25 October 1984 — Section 7: Pipe Threads, General Purpose
• ANSI/ASME B1.20.1-1983 — Pipe Threads, General Purpose (Inch). Named by FED-STD-H28/7A (1984) §4; not consulted directly.
• ANSI/ASME B1.13M-1983 — Metric Screw Threads: M Profile, and ANSI B1.21M-1978 — Metric Screw Threads: MJ Profile. Named by FED-STD-H28/21B (1984) §4.1; not consulted directly.
• ASME B1.3M-1992 — Screw Thread Gaging Systems for Acceptability: Inch and Metric Screw Threads. Named by FED-STD-H28/20B (1994) §5.1.1.1; not consulted directly.