Part Type
Custom Thread, Plug and Ring Gauges
MI CNC Parts quotes custom thread, plug and ring gauges made to the class on your print: GO/NO-GO gauges such as a thread plug for a 2B hole at an odd pitch, a left-hand 3A thread ring, or a pin gauge at a size between the standard steps. Send the thread callout, or the part print with the feature marked, and a person quotes it in 24-48 hours. One gauge is a normal order.
Updated October 2026
Example part, modeled for illustration.
Start Here
Get a Quote in 24-48 Hours
Send us your files and we’ll quote it, usually within 24-48 hours.
Need an NDA before you send files? Ask for one and it’s sent the same day.
Commercial work only. We don’t quote firearms, DoD, ITAR, or EAR-controlled parts.
Or email your print to evan@micncparts.com.
Thread plugs and rings, setting plugs, pin and plug gauges, quoted from your callout.
What should I send for a gauge quote?
The thread callout, written the way the print writes it (1/2-13 UNC-2B, M6x1-6g, 3/4-16 UNF-3A-LH, 2.500-12 UNJ-3A), and whether you need the plug, the ring, or both.
For a pin, plug or ring gauge: the size, the class, and whether it is the GO or the NO GO. If all you have is the part tolerance, send that and the quote proposes a class.
If the thread is plated after it is cut, the plating thickness range.
For an adjustable thread ring, whether you already own its setting plug.
For a keyway, spline or NPT gauge, the print with the feature fully dimensioned.
The quantity, and any material, hardness or plating the gauge print calls for.
Need a calibration certificate with the gauge? Tell us when you send the print, and we’ll confirm before you order.
No gauge print is fine: the part print with the feature and its limits marked is enough. Any file type is accepted; STEP plus PDF is ideal.
Which gauge checks which feature?
Each gauge is a pair, or one gauge with two ends: the GO member checks one limit of the feature and the NO GO checks the other, so a part passes or fails without anyone taking a reading (source 1). Thread gauges work the same way and check the thread form as well as its size, but they give no pitch diameter number (source 2).
| Feature on the part | Gauge | On a good part |
|---|---|---|
| Tapped hole | Thread plug, GO and NO GO | GO threads in, NO GO does not |
| External thread | Thread ring, GO and NO GO | GO threads on; NO GO may start on the end threads but goes no more than three complete turns |
| Adjustable thread ring | Setting plug, same size and class | Sets the ring; never used on a part |
| Plain hole | Plug or pin gauge, GO and NO GO | GO enters, NO GO does not |
| Shaft or outside diameter | Plain ring or snap gauge | Checked between its upper and lower limit |
| Hole pattern, profile or location | Checking fixture | Checked on the fixture against the print |
How do I match a thread gauge to the part’s thread class?
Order the gauge in the class on the print. A 1/4-20 UNC 2A external thread is checked with a 1/4-20 UNC 2A ring set, and a 2B tapped hole with a 2B plug (source 3). In Unified inch threads, A marks an external thread and B an internal one: 1A, 2A and 3A are external, 1B, 2B and 3B internal. Class 2 is the most common fit, and class 3 is used where the work needs closer tolerances (source 4).
Metric threads follow the same rule with a different code, lowercase for external and uppercase for internal (source 5). An M6x1-6g screw takes a 6g ring, and a 6H hole is checked with a 6H plug (source 3).
Gauges for Unified inch threads (UN, UNR and UNJ) are covered by ASME B1.2, and gauges for M-series metric threads by ASME B1.16M (sources 6 and 7).
UNJ threads follow the same rule. Thread rings are quoted in UNJ-3A and thread plugs in UNJ-3B, so a 2.500-12 UNJ-3A external thread takes a 2.500-12 UNJ-3A ring and a 1.3125-12 UNJ-3B hole takes a 1.3125-12 UNJ-3B plug. Write UNJ in the callout, not UN, so the gauges are quoted to the UNJ limits on your print.
An adjustable thread ring is set to size with a setting plug of the same size and class, and a setting plug is never used to check a part (sources 3 and 8). If you are replacing a ring and already own its setting plug, say so; if not, the quote can include one.
What gage class does a pin or plug gauge need?
Gagemaker classes are one-direction tolerances. A gauge is made plus from its marked size or minus from it, never both, so a class Z plus pin marked 0.2500 in measures 0.2500 to 0.2501 in, and a class Z minus pin 0.2499 to 0.2500 in (source 9).
A GO plug carries plus tolerance and a NO GO plug minus. A GO that enters then proves the hole is above its minimum, and a NO GO that stops proves it is below its maximum (source 9).
To pick the class, Meyer Gage gives 10 percent of the product tolerance to the gauges, half to each member. Their example: a 0.250 in hole with 0.004 in of tolerance leaves 0.0004 in for the gauges, so the GO is 0.250 in plus 0.0002 and the NO GO 0.254 in minus 0.0002, which is class ZZ (source 9).
We quote to the class on your print. The chart below is for reference.
| Diameter (in) | XX | X | Y | Z | ZZ |
|---|---|---|---|---|---|
| 0.0009-0.8250 | 0.000020 | 0.000040 | 0.000070 | 0.000100 | 0.000200 |
| 0.8251-1.5100 | 0.000030 | 0.000060 | 0.000090 | 0.000120 | 0.000240 |
| 1.5101-2.5100 | 0.000040 | 0.000080 | 0.000120 | 0.000160 | 0.000320 |
| 2.5101-4.5100 | 0.000050 | 0.000100 | 0.000150 | 0.000200 | 0.000400 |
| 4.5101-6.5100 | 0.000065 | 0.000130 | 0.000190 | 0.000250 | 0.000500 |
| 6.5101-9.0100 | 0.000080 | 0.000160 | 0.000240 | 0.000320 | 0.000640 |
| 9.0101-12.2600 | 0.000100 | 0.000200 | 0.000300 | 0.000400 | 0.000800 |
What are gauges made from?
Meyer Gage lists its pin gauges in 52100 bearing steel at 60-62 HRC, centerless lapped, with class Z pins at 8 microinch or better and class X at 2 microinch or better (sources 10 and 11). Vermont Gage lists its thread rings and setting plugs in tool steel at 60-62 HRC and its working thread plugs chrome-plated at 70-72 HRC (sources 8 and 12). If your gauge needs a different material, hardness or plating, put it on the print and the quote follows the print.
Whether a part needs single gauges or a full checking fixture is covered on check fixture vs gauge, and the fixture itself on check and inspection fixtures.
Sources
All accessed 27 September 2026. Where a figure here differs from your print, the print governs.
1. Wikipedia, "Go/no-go gauge": en.wikipedia.org/wiki/Go/no-go_gauge (plug gauge GO and no-go ends; snap gauge anvils set to the upper and lower limit).
2. The Gage Store, "How to Use Thread Gages": thegagestore.com/docs/HowToUse/Thread_Gages/The_Gage_Store-Measurement_Tools_and_Accessories-How_to_Use_Thread_Gages.htm (GO plug in and NO GO out; NO GO ring no more than three complete turns; functional check, no pitch diameter reading).
3. Mike Oddy, "Thread Classes and Setting Plug Gages", Quality Magazine, 1 October 2019: qualitymag.com/articles/95710 (the product thread class orders the gauge; 2A ring, 2B and 6H plug examples; setting plugs set adjustable rings and never inspect product).
4. Wikipedia, "Unified Thread Standard": en.wikipedia.org/wiki/Unified_Thread_Standard (classes 1A-3A external, 1B-3B internal; class 2 most common, class 3 closer tolerances; UNS special threads).
5. Wikipedia, "ISO metric screw thread": en.wikipedia.org/wiki/ISO_metric_screw_thread (external tolerance classes lowercase, internal uppercase).
6. ASME B1.2-2025, "Gages and Gaging for Unified Inch Screw Threads (UN, UNR, and UNJ)": asme.org.
7. ASME B1.16M-1984 (S2021), "Gages and Gaging for Metric M Screw Threads": asme.org.
8. Vermont Gage, "Thread Ring & Setting Plug Gages": vermontgage.com/gaging-products/thread-gages/thread-ring-and-setting-plug-gages (setting plugs set adjustable rings; tool steel 60/62 Rc).
9. Meyer Gage Company via Newman Tools, "The ABC’s of Pin Plug Gages, and the 10 percent gauge rule": newmantools.com/meyer/pluggage_ABC.htm (tolerance chart, plus and minus gage tolerance, GO plus and NO-GO minus, the 10% rule and its 0.250 in example).
10. Meyer Gage via Newman Tools, "Class X Pin Gages": newmantools.com/meyer/x.htm (52100 bearing steel, 60-62 Rockwell C, centerless lapped, 2 microfinish or better).
11. Penn Tool Co., "Meyer Steel Pin Gage Sets" (class Z): penntoolco.com/meyer-steel-pin-gage-sets/ (52100 bearing steel, centerless lapped, 8 microfinish or better, 60-62 Rockwell C).
12. Vermont Gage, "Working Thread Plug Gages": vermontgage.com/gaging-products/thread-gages/working-thread-plug-gages (chrome, 70/72 Rc).
13. Mike Oddy, "Pre-Plate Threads: 60° Thread Form", Quality Magazine, 3 May 2024: qualitymag.com/articles/97967 (pitch diameter changes 4x the plating thickness; pre-plate formulas).
14. Gage Crib Worldwide, ASME B1.7 definition of a left-hand thread: ring-plug-thread-gages.com/ASME-B1.7-Terms-Definitions/left-hand-thread.htm (screwed in or on counterclockwise; all designated LH).
15. Vermont Gage, "NPT Taper Pipe Thread Inspection": vermontgage.com/support/detail/npt-taper-pipe-thread-inspection (L1 plug and ring check hand-tight engagement; pass within one turn of flush; ASME B1.20.1-2013).
Questions
Before you send a job.
01 How do I order a left-hand thread ring gauge?
Put LH at the end of the callout, for example 3/4-16 UNF-3A-LH, and order the GO and NO GO rings in that class. ASME B1.7 defines a left-hand thread as one screwed in or on counterclockwise and designates every one of them LH, so paperwork without it describes a right-hand ring.
02 Can a thread gauge be made for a non-standard size like 2.360-18?
Yes. A diameter and pitch outside the standard series is a special, or UNS, thread, found more on nuts, tapped holes and threaded ODs than on bolts. There is no standard table to read its limits from, so send the pitch diameter limits from your print, or the major diameter, pitch and class, and the gauges are quoted to those numbers. The same goes for the thread on a custom fastener.
03 Can you quote a UNJ thread ring or plug gauge?
Yes. UNJ thread rings are quoted in class 3A and UNJ thread plugs in class 3B, GO and NO GO, to the class on your print. Send the callout as the print writes it, for example 2.500-12 UNJ-3A for a ring or 1.3125-12 UNJ-3B for a plug, and say whether you need the GO, the NO GO or both.
04 How is a thread gauge sized for a part that gets plated?
To the pre-plate limits, not the finished ones. On a 60 degree thread the pitch diameter changes by four times the plating thickness, so 0.0002 in of plating moves it 0.0008 in. For an external thread, the maximum pre-plate pitch diameter is the maximum after-plate figure less four times the maximum plating, and the minimum is the minimum after-plate figure less four times the minimum plating. Quality Magazine warns that one offset applied to both limits accepts bad threads or scraps good ones.
05 Can you quote a keyway, spline or NPT gauge?
Send the print and you get a yes or no before you order, with the price if it is a yes. One difference to expect on NPT: a taper pipe thread is not checked with a GO and NO GO pair but with an L1 plug or L1 ring for hand-tight engagement, and the part passes when it sits within one turn of flush with the notch on the plug or the small-end face of the ring. That is the ASME B1.20.1 practice as Vermont Gage describes it (source 15).
06 What is the class Z tolerance on a 1.2505 in plug gauge?
Class Z widens with size. From 0.8251 to 1.5100 in it is 0.000120 in, not the 0.000100 in of the smaller sizes, so a 1.2505 in class Z plus pin runs 1.2505 to 1.25062 in. Order it by size, class, and GO or NO GO.
07 How long does a custom thread gauge take?
The quote comes back in 24-48 hours. Rush: 1-2 weeks. Standard: 4-6 weeks (typical). Extended: 6-8 weeks, for jobs that aren’t time-sensitive. Rush 1-2 weeks applies to machined parts with no secondary operations. Heat treat, grinding, EDM and finishing add time, and the quote gives the real date. A hardened and ground gauge has two of those steps. Lead time starts at order approval. Put your need date in the part description and the quote answers it. How long custom machined parts take explains where the weeks go.
Get Started
Send us your files.
We’ll take it from there.
A quote in 24-48 hours, no minimums. Like the quote, and we’ll make your parts and ship them to you.
Get a Quote