ACME threads are a power-transmission thread form used in lead screws, jacks, vises, and linear actuators. This guide covers the ACME thread standard, size chart, tolerance classes, gauge types, and how to select the correct GO/NO-GO gauge for General Purpose and Stub ACME threads per ASME B1.5 and B1.8.
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What Is an ACME Thread?
ACME threads are a standardized power transmission thread form defined by ASME B1.5. They replaced the older square thread form in most applications because they are stronger, easier to manufacture, and allow the use of split nuts for disengagement. The defining characteristic of ACME threads is their 29° included angle — different from the 60° angle used by metric and UN threads.
Need GO/NO-GO gauges for this thread?
GageTools manufactures GO/NO-GO thread plug gauges from USD 20 and thread ring gauges from USD 30. MOQ 1 piece, stock sizes ship within 72 hours, and a calibration report or ISO 17025 certificate is available on request.
Thread Plug GaugesThread Ring GaugesRequest a QuoteACME vs Metric vs UN vs Trapezoidal Threads
ACME is the American power thread standard. The European equivalent is the Trapezoidal thread (DIN 103 / ISO 2903) with a 30° angle. Metric and UN threads use 60° and are fastening threads, not power transmission threads.
| Thread Form | Angle | Standard | Primary Use | Region |
|---|---|---|---|---|
| ACME | 29° | ASME B1.5 | Power transmission, lead screws | USA / Canada |
| Trapezoidal (Tr) | 30° | DIN 103 / ISO 2903 | Power transmission, lead screws | Europe / Global |
| Metric (ISO) | 60° | ISO 261 / DIN 13 | Fastening, general assembly | Global |
| UN (UNC/UNF) | 60° | ANSI/ASME B1.1 | Fastening, general assembly | USA / Canada |
ACME Thread Size Chart — General Purpose
Standard: ASME B1.5. All dimensions in inches. Thread depth = 0.5 × pitch. Default tolerance class: 2G.
| Size | TPI | Pitch (in) | Major Ø (in) | Pitch Ø (in) | Minor Ø (in) | Thread Depth (in) | Gauge |
|---|---|---|---|---|---|---|---|
| 1/4-16 | 16 | 0.0625 | 0.2500 | 0.2188 | 0.1875 | 0.0313 | Quote |
| 5/16-14 | 14 | 0.0714 | 0.3125 | 0.2768 | 0.2411 | 0.0357 | Quote |
| 3/8-12 | 12 | 0.0833 | 0.3750 | 0.3333 | 0.2917 | 0.0417 | Quote |
| 1/2-10 | 10 | 0.1000 | 0.5000 | 0.4500 | 0.4000 | 0.0500 | Quote |
| 5/8-8 | 8 | 0.1250 | 0.6250 | 0.5625 | 0.5000 | 0.0625 | Quote |
| 3/4-6 | 6 | 0.1667 | 0.7500 | 0.6667 | 0.5833 | 0.0833 | Quote |
| 7/8-6 | 6 | 0.1667 | 0.8750 | 0.7917 | 0.7083 | 0.0833 | Quote |
| 1″-5 | 5 | 0.2000 | 1.0000 | 0.9000 | 0.8000 | 0.1000 | Quote |
| 1-1/4″-5 | 5 | 0.2000 | 1.2500 | 1.1500 | 1.0500 | 0.1000 | Quote |
| 1-1/2″-4 | 4 | 0.2500 | 1.5000 | 1.3750 | 1.2500 | 0.1250 | Quote |
| 1-3/4″-4 | 4 | 0.2500 | 1.7500 | 1.6250 | 1.5000 | 0.1250 | Quote |
| 2″-4 | 4 | 0.2500 | 2.0000 | 1.8750 | 1.7500 | 0.1250 | Quote |
| 2-1/2″-3 | 3 | 0.3333 | 2.5000 | 2.3333 | 2.1667 | 0.1667 | Quote |
| 3″-2 | 2 | 0.5000 | 3.0000 | 2.7500 | 2.5000 | 0.2500 | Quote |
| 4″-2 | 2 | 0.5000 | 4.0000 | 3.7500 | 3.5000 | 0.2500 | Quote |
ACME Thread Tolerance Classes — 2G, 3G, 4G, 5G, 6G
ACME General Purpose threads use a class system designated by a number and the letter G. Higher numbers indicate tighter tolerances and closer fits. Class 2G is the most commonly used for general engineering applications.
| Class | Fit Type | Typical Use | Notes |
|---|---|---|---|
| 2G | Loose | General purpose machinery, lead screws | Most widely used · Allows for lubrication clearance |
| 3G | Medium | Higher accuracy applications | Tighter than 2G · Less backlash |
| 4G | Close | Precision positioning, measuring equipment | Tight fit · Minimal backlash |
| 5G / 6G | Very close | High precision instruments | Tightest fits · Special manufacturing required |
Stub ACME Threads — ASME B1.8
Stub ACME threads have a shallower thread depth than General Purpose ACME — approximately 30% less depth. They are defined by ASME B1.8 and are used when the full ACME thread depth would leave insufficient material in the root of the thread, such as in thin-wall parts or hollow shafts.
| Property | General ACME | Stub ACME |
|---|---|---|
| Thread angle | 29° | 29° |
| Thread depth | 0.5 × pitch | 0.3 × pitch |
| Standard | ASME B1.5 | ASME B1.8 |
| Designation example | 1/2-10 ACME-2G | 1/2-10 Stub ACME-2G |
How to Select the Correct ACME Thread Gauge
ACME thread gauges follow the same GO/NO-GO principle as metric and UN thread gauges. You need the following information from your drawing to order correctly.
| Inspecting | Order this gauge |
|---|---|
| Internal ACME (nut/housing) | ACME Plug Gauge GO/NO-GO |
| External ACME (lead screw) | ACME Ring Gauge GO/NO-GO |
| Internal Stub ACME | Stub ACME Plug Gauge GO/NO-GO |
| External Stub ACME | Stub ACME Ring Gauge GO/NO-GO |
Request an ACME Thread Gauge Review
For an accurate quotation, send the ACME thread form, nominal diameter, threads per inch, class, plug or ring gauge requirement, GO/NO-GO configuration, quantity and any certificate requirement.
Need a made-to-drawing solution? See our custom gauge manufacturing options.
General-Purpose vs Centralizing Acme Classes
ASME B1.5 defines two families of Acme threads, and the family decides how the gauges are built and what they check.
| Feature | General-purpose (G) | Centralizing (C) |
|---|---|---|
| Classes | 2G, 3G, 4G | 2C, 3C, 4C (plus tighter 5C and 6C) |
| How the thread centres | Clearance on major and minor diameters; flanks carry the load | Close fit on the major diameter keeps screw and nut concentric |
| Typical use | Clamps, vices, jacks, general power screws | Lead screws, machine tools, where wobble must be limited |
| Gauging focus | Pitch diameter and flank form | Pitch diameter plus closer control of major diameter |
Higher class numbers mean tighter tolerances. A 2G thread is the common default; 3G and 4G reduce backlash. General-purpose and centralizing gauges are not interchangeable, even for the same diameter and pitch, so the class must be clear on the drawing.
Acme vs Trapezoidal (Tr) Threads
Acme and metric trapezoidal threads look very similar, and parts are often mistaken for each other in export trade. They are not interchangeable.
| Feature | Acme (ASME B1.5) | Trapezoidal Tr (ISO 2904 / DIN 103) |
|---|---|---|
| Included angle | 29° | 30° |
| Size units | Inch diameter, threads per inch | Millimetre diameter and pitch |
| Example designation | 1-5 ACME-2G | Tr 24×5-7H |
| Tolerance classes | 2G–4G, 2C–6C | 7H/7e, 8H/8e and others |
The 1° angle difference and inch versus metric pitch mean a Tr nut may start on an Acme screw but will bind or wear quickly. Always gauge with the system on the drawing. For metric trapezoidal sizes and gauges, see our trapezoidal thread gauge guide.
Common Acme Sizes and What to Gauge
ASME B1.5 lists preferred diameter and pitch combinations for single-start Acme threads. These are the sizes most often requested for plug and ring gauges.
| Nominal diameter (in) | Threads per inch | Pitch (in) |
|---|---|---|
| 1/4 | 16 | 0.0625 |
| 3/8 | 12 | 0.0833 |
| 1/2 | 10 | 0.1000 |
| 5/8 | 8 | 0.1250 |
| 3/4 | 6 | 0.1667 |
| 1 | 5 | 0.2000 |
| 1-1/4 | 5 | 0.2000 |
| 1-1/2 | 4 | 0.2500 |
| 2 | 4 | 0.2500 |
Acme GO gauges check that the thread assembles over its full length and has enough flank clearance; NOT GO gauges check that the pitch diameter is not beyond its limit. Because Acme threads are coarse and long, lead error and drunkenness along the length can stop a GO gauge even when the pitch diameter is correct at one point, so inspect the full engagement length rather than the first few turns.
Gauging Multi-Start Acme Threads
Many Acme lead screws are multi-start to increase travel per revolution. The key relationship is lead = pitch × number of starts. A two-start thread with 0.2 in pitch has a 0.4 in lead.
- Gauges must match the number of starts. A single-start gauge with the same pitch will not enter a multi-start thread correctly.
- Specify pitch, lead and starts. ASME B1.5 designations show all three, for example 1.000-0.2P-0.4L-ACME-2G, which is a two-start thread.
- Index errors matter. Uneven spacing between starts can make a part reject even if each start is correct individually, so multi-start gauges are made with controlled indexing.
- Allow extra lead time. Multi-start gauges are usually custom; standard custom lead time is 10–15 working days.
Acme Gauge Ordering Checklist
- Nominal diameter and threads per inch, or pitch and lead for multi-start threads.
- Thread family and class: general-purpose (2G, 3G, 4G) or centralizing (2C–6C).
- Internal (plug gauge) or external (ring gauge), or both.
- Right-hand or left-hand thread.
- Length of engagement or any special depth requirement.
- Calibration certificate needs, such as ISO/IEC 17025.
- A drawing if the thread is non-standard or modified; we can make custom gauges from drawings with an MOQ of 1 piece.
For a closer look at the 29° form itself — basic proportions, general-purpose vs centralizing vs Stub Acme, and how Acme compares with trapezoidal, square and buttress threads — see our Acme thread profile guide.
Frequently Asked Questions
More Specification Points to Confirm
- Acme (29°) vs. Metric Trapezoidal Tr (30°): Critical Differences: While both Acme and Trapezoidal threads feature a trapezoidal profile designed for power transmission, their geometric standards differ and are not interchangeable: Acme Threads (ASME B1.5 / B1.8): Utilize a 29° thread angle with Imperial inch dimensions (e.g., 1″-5 Acme).
- Clarify Single-Start vs. Multi-Start Pitch and Lead: The single most common mistake when sourcing trapezoidal thread gauges is confusing Pitch (distance between adjacent thread crests) with Lead (axial distance moved in one 360° revolution).
- Choose General Purpose vs. Centralizing Acme Classes: Selecting the wrong fit class results in leadscrew alignment issues or severe mechanical friction: General Purpose Acme (2G, 3G, 4G): Class 2G is the universal B2B standard for commercial power threads.
- Pair Acme Ring Gauges with Truncated Master Plugs: Because external Acme leadscrews require rigorous quality control, adjustable Acme thread ring gauges must be set using matching truncated master setting plug gauges.
- Verify Missing Prints or Worn Leadscrew Samples: If you are replacing a worn shop-floor Acme gauge or lack original factory drawings, check our engineering protocol on replacing thread gauges without a drawing .