Thread Standards — Family Index
1. At a glance
Screw threads are the universal mechanical interface — fastening, sealing, motion, and adjustment. The world runs on two parallel inch and metric systems that meet at the same 60° flank geometry but diverge on sizing, tolerance grades, and pitch tables. A practicing engineer must move fluently between them and recognise a handful of specialty profiles (ACME, trapezoidal, NPT, BSP, UNJ/MJ, buttress, BA, UNM) that exist for power transmission, sealing, fatigue resistance, or historical reasons.
| Family | Flank angle | Use | Standard |
|---|---|---|---|
| UN (UNC/UNF/UNEF/UNS) | 60° | General fasteners — inch | ASME B1.1 |
| UNJ | 60° | Aerospace fatigue — inch | ANSI/ASME B1.15 |
| M (metric ISO) | 60° | General fasteners — metric | ISO 261, 262, 965 |
| MJ | 60° | Aerospace fatigue — metric | ISO 5855 / ASME B1.21 |
| NPT / NPTF | 60° | Tapered pipe — inch | ASME B1.20.1 / B1.20.3 |
| BSPT (R) | 55° Whitworth | Tapered pipe — metric/UK | ISO 7-1 |
| BSPP (G) | 55° Whitworth | Parallel pipe — metric/UK | ISO 228-1 |
| ACME (2G/3G/4G) | 29° | Power transmission — inch | ANSI B1.5 |
| Stub-ACME | 29° | Short-thread power — inch | ANSI B1.8 |
| Trapezoidal (Tr) | 30° | Power transmission — metric | ISO 2902 / DIN 103 |
| Buttress | 7° / 45° asym. | One-way high axial load | ANSI B1.9 / DIN 513 |
| Whitworth (BSW/BSF) | 55° | Legacy UK | BS 84 (withdrawn) |
| BA (British Association) | 47.5° | Historical instruments | BS 93 |
| UNM | 60° | Miniature inch/metric 0.30–1.40 mm | ISO 1501 / ANSI B1.10 |
Everything else is a class-of-fit, length, hand (right/left), or starts-count variant of one of those.
2. Inch threads — Unified (UN)
The Unified Thread Standard (UTS) is the post-WWII reconciliation of US National (USS / NS / NF) and British Whitworth into a single 60° flank inch system. Defined in ASME B1.1. Sizes nominal from 0-80 (0.060″ major dia, 80 TPI) up to 6-4 (six-inch major, 4 TPI). Threads per inch (TPI) is the inch equivalent of pitch.
Series:
- UNC — Unified Coarse. Default for ductile-iron/steel/aluminium fastening. Examples: 1/4-20, 3/8-16, 1/2-13, 5/8-11, 3/4-10, 1-8.
- UNF — Unified Fine. Better strip resistance in thin material, finer adjustment, higher tensile area. Examples: 1/4-28, 3/8-24, 1/2-20, 5/8-18.
- UNEF — Unified Extra-Fine. Thin-wall tubing, instrument bezels, fine adjustment. Example: 1/4-32, 1/2-28.
- UNS — Unified Special. Non-standard pitch on a standard diameter, or vice-versa. Tabulated in B1.1 Appendix; use only when a standard series will not work.
- UNJ — controlled root radius (0.150 × P min) for high-cycle fatigue. Mandatory for primary aerospace structure. See §6.
Inch sizes below 1/4″ use numbered gauges 0–12 (e.g. 10-32). Above 1/4″, fractional inches.
3. Metric ISO
Metric threads are defined by the basic ISO 68-1 60° profile. ISO 261 lists the general-purpose pitch series; ISO 262 is the preferred selection for fasteners; ISO 965-1/2/3 sets tolerances.
Designation: M + nominal major dia × pitch, e.g. M10×1.5.
Coarse-pitch examples (default unless otherwise specified): M1.6×0.35, M2×0.4, M2.5×0.45, M3×0.5, M4×0.7, M5×0.8, M6×1.0, M8×1.25, M10×1.5, M12×1.75, M16×2.0, M20×2.5, M24×3.0, M30×3.5, M36×4.0, M42×4.5, M48×5.0, M64×6.0, M100×6.0.
Fine pitches (call out explicitly because coarse is implied if pitch is omitted): M8×1.0, M10×1.25 or M10×1.0, M12×1.5 or M12×1.25, M16×1.5, M20×1.5, M24×2.0. Fine pitch raises tensile stress area, reduces self-loosening, and is preferred for thin-wall tubing and instruments.
MJ is the metric J-series equivalent of UNJ (§6) — same M profile but with a controlled, larger root radius.
4. Pipe threads
Pipe threads are a separate world because they exist primarily to seal rather than fasten. Two unrelated families dominate.
4.1 US — NPT and friends (ASME B1.20.1, B1.20.3)
| Series | Form | Use |
|---|---|---|
| NPT | Tapered, 60° flank, 1°47′24″ taper per side (1:16 included) | General pipe — needs PTFE tape or pipe-dope sealant |
| NPTF | Dryseal — same taper, controlled truncation at crest and root for metal-to-metal seal | Hydraulic and fuel without sealant |
| NPSC | Straight Coupling | Mechanical joint at coupling |
| NPSL | Straight Locknut | Locknut on tapered male |
| NPSM | Straight Mechanical | Mechanical, non-sealing |
Pipe sizes are nominal pipe size (NPS) not OD — e.g. “1/2-14 NPT” has 14 TPI and an actual OD around 0.840″. Always specify male/female on a drawing; thread engagement length matters because the taper means a too-deep tap leaves the joint at less than design wall thickness.
4.2 ISO/UK — BSP (ISO 7-1, ISO 228-1)
| Series | Form | Designator | Use |
|---|---|---|---|
| BSPT | Tapered, 55° Whitworth, 1:16 taper | R (external), Rc (internal taper), Rp (internal parallel mating R) | Sealed pipe — non-US |
| BSPP | Parallel, 55° Whitworth | G | Mechanical joint; seal by bonded washer, O-ring boss, or face-seal |
BSP and NPT are not interchangeable — different flank angle (55° vs 60°), different tapers, different TPI at the same nominal size. A 1/2 NPT fitting in a 1/2 BSP port will leak or strip.
5. Power-transmission threads
When the thread exists to move a load rather than clamp, the 60° flank is inefficient (high radial reaction force). Specialty trapezoidal profiles minimise friction and improve nut wear life.
5.1 ACME (inch — ANSI B1.5)
- 29° flank. Three classes — 2G (general), 3G (medium fit), 4G (precise).
- Centralizing ACME (CLAS) — pitch and minor diameter both controlled so the nut centers on the screw; required where side-load on the leadscrew is significant.
- Stub-ACME (ANSI B1.8) — half-height ACME for shallow-thread applications (jacks, clamps).
- Examples: 1/2-10 ACME-2G, 1-1/2-4 ACME-CLAS, 2-12 Stub-ACME.
5.2 Trapezoidal (metric — ISO 2902/4, DIN 103)
- 30° flank. Designated
Tr+ major × pitch, e.g. Tr 40×7. Multi-start: Tr 40×14(P7) = 14 mm lead, 7 mm pitch, 2-start. - Tolerance fields per ISO 2903: external 7e/8e/9c, internal 7H/8H/9H.
- Metric replacement for ACME in non-US machinery — leadscrews on lathes, presses, automotive jacks.
5.3 Buttress (ANSI B1.9 / DIN 513)
- Asymmetric flanks — 7° on the load side, 45° on the back. Carries axial load in one direction with the radial efficiency of an ACME (because the load flank is nearly perpendicular to the axis).
- Examples: 1.5-6 Push-Buttress, S 40×7 (DIN 513 sägengewinde).
- Use cases: artillery breech, vise screws, pile clamps, large hydraulic presses, oilfield casing connections (modified versions).
5.4 Sawtooth / Rope
Variants of buttress with different flank ratios for very specific industries — petroleum casing threads (API 5B Buttress), heavy lifting eye screws.
6. Aerospace threads — UNJ and MJ
Primary aerospace structure runs almost exclusively on J-series threads because the controlled, larger root radius dramatically extends fatigue life.
- UNJ (inch) — ANSI/ASME B1.15, formerly MIL-S-8879. Min root radius 0.150 × P, max 0.180 × P. UN flanks otherwise identical.
- MJ (metric) — ISO 5855 / ASME B1.21. Same idea on the M profile. MJ tolerance is tighter than M.
- STI — Screw Thread Insert. Tap drill is oversized to accept a Heli-Coil (or Recoil, Time-Sert, KATO) wire/solid insert that presents a UN/M internal thread to the bolt. Designation:
M8×1×100/100/12 STI. Use when:- Parent material is soft (Al, Mg, plastic) and would strip under torque or thermal cycling.
- Tapped hole sees frequent re-assembly.
- Repair of a stripped thread.
- NAS / MS / AN fastener callouts (NAS6606H6, MS21250-04014) embed thread size, length, material, locking element, and inspection class — far more than a B&S spec.
7. Whitworth (BSW / BSF) — historical
British Standard Whitworth, Joseph Whitworth 1841 — the first standardised thread. 55° flank, rounded crest and root. Replaced by ISO metric for new UK design in 1965, but survives in:
- Classic British motorcycles (Triumph, BSA, Norton pre-1968).
- Camera and tripod mounts (1/4-20 UNC and 3/8-16 UNC are actually ISO-equivalents, but old 3/8 BSW tripods exist).
- Plumbing — BSP is a direct descendant (55° flank, parallel or taper).
BSF = British Standard Fine — finer-pitch Whitworth. BSC = British Standard Cycle.
8. Micro / instrument threads
- UNM (ASME B1.10 / ISO 1501) — Unified Miniature. Sizes 0.30 mm through 1.40 mm in roughly 0.05 mm steps. Used in watches, micro-optics, medical implants. 60° flank.
- BA (British Association, BS 93) — 47.5° flank, sizes 0BA (6.0 mm major) to 22BA (0.25 mm). Almost entirely replaced by M, but persists in vintage optical instruments, model engineering, and antique electrical equipment.
- For very small fastener counts in modern design, M1, M1.2, M1.4, M1.6, M2, M2.5, M3 cover the field — BA is essentially obsolete for new work.
9. Class of fit
9.1 Inch (ASME B1.1)
| Class | Use |
|---|---|
| 1A / 1B | Loose — quick assembly, dirty conditions, plated threads |
| 2A / 2B | Standard — 90% of commercial fasteners. Most stock screws are 2A. |
| 3A / 3B | Close — structural, aerospace, high-temperature, where back-off resistance matters |
A = external (screw / bolt), B = internal (nut / tapped hole). Allowance (clearance) decreases as class number rises.
9.2 Metric (ISO 965-1, 965-2)
Tolerance class is two parts: pitch-diameter tolerance grade (digit) + fundamental deviation position (letter). Lowercase = external, uppercase = internal.
| Common class | Meaning |
|---|---|
| 4h | Close external |
| 6g | Standard external (commercial bolt) |
| 6h | Close external — no allowance |
| 6H | Standard internal (nut, tapped hole) |
| 4H6H | Close internal (4H pitch dia, 6H minor) |
A double designation like M10×1.5 - 6g/6H specifies the screw–nut fit pair.
9.3 Equivalence
- 1A ≈ 8g
- 2A ≈ 6g, 2B ≈ 6H (standard commercial fit)
- 3A ≈ 4h, 3B ≈ 5H (close fit)
10. Geometry math (UN and M, 60° flank)
Let:
P= pitch (mm or 1/TPI inches)D= nominal major diameterD₁= minor diameter (root)D₂= pitch diameter (effective)H= full thread height (sharp V)n= number of startsL= lead (axial advance per revolution) =n × P
H = (√3 / 2) × P ≈ 0.866 × P (sharp V height)
D₁ = D − 1.0825 × P (basic minor diameter, UN/M)
D₂ = D − 0.6495 × P (basic pitch diameter, UN/M)
λ = arctan( L / (π × D₂) ) (helix angle at pitch line)
Tensile stress area (used in proof-load and tensile strength of a bolt):
A_t = (π / 4) × ( (D − 0.9382 × P) )² (ISO 898, metric)
A_t = 0.7854 × ( D − 0.9743 / n )² (UN, n = TPI)
ACME (29° flank): H = 0.5 × P, D₂ = D − 0.5 × P. Trapezoidal Tr (30°): H₁ = 0.5 × P + a_c where a_c is the clearance at root/crest (DIN 103).
11. Tap-drill sizing
Rule of thumb for ≈75% thread engagement (the common compromise between strength and tap life):
Tap drill (mm) = D − P (metric, ≈75%)
Tap drill (in) = D − 1/n (UN, ≈75%)
For ≈60% engagement (long taps in tough material, deeper holes): D − 1.3 × P (metric).
Reference table (≈75%):
| Thread | Tap drill | Thread | Tap drill |
|---|---|---|---|
| M3×0.5 | 2.5 mm | 4-40 | #43 (0.089″) |
| M4×0.7 | 3.3 mm | 6-32 | #36 (0.107″) |
| M5×0.8 | 4.2 mm | 8-32 | #29 (0.136″) |
| M6×1.0 | 5.0 mm | 10-24 | #25 (0.150″) |
| M8×1.25 | 6.8 mm | 10-32 | #21 (0.159″) |
| M10×1.5 | 8.5 mm | 1/4-20 | #7 (0.201″) |
| M12×1.75 | 10.2 mm | 1/4-28 | #3 (0.213″) |
| M14×2.0 | 12.0 mm | 5/16-18 | F (0.257″) |
| M16×2.0 | 14.0 mm | 3/8-16 | 5/16″ (0.3125″) |
| M20×2.5 | 17.5 mm | 1/2-13 | 27/64″ (0.4219″) |
Cast iron and brittle materials prefer 60–70% engagement; aluminium and brass tolerate 75–80%. Stainless and titanium use coolant-thru taps and accept galling risk near 75%.
12. Self-tapping and thread-forming screws
Cold-form threads in a pre-drilled hole — no separate tapping operation. Designations from ANSI/ASME B18.6.4 (metallic) and DIN 7500 (forming, metric).
| Type | Profile | Material | Notes |
|---|---|---|---|
| A | Coarse, gimlet point | Thin sheet metal | Withdrawn, replaced by AB |
| AB | Coarse, gimlet point, B-thread | Sheet metal, plastic | Common sheet-metal screw |
| B | Machine-screw thread, blunt | Sheet metal, castings | Higher pull-out than A |
| T | Hardened, fluted cutting edge | Thicker stock — actually cuts a chip | Functionally a self-cutting tap |
| Type 23 / 25 (Plastite) | Trilobular cross-section | Plastic (Type 25 = wider thread for soft plastic) | Cold-forms, no chip |
| Taptite / Powerlok / Duro-PT (DIN 7500) | Trilobular metric | Steel, aluminium | Cold-forms, work-hardens hole — excellent vibration resistance |
| Drill-and-tap (TEK) | Drill point + thread | Steel sheet & light-gauge structural | One-step install through metal |
Forming taps (no flutes, no chips) are the production-tool counterpart for tapped holes in ductile materials — leaves a stronger, work-hardened thread and produces no chip to clear.
13. Sealing — taper vs straight + sealant
This is the single most common source of leaks in hydraulic and pneumatic systems. A taper thread seals on the flanks under wedge action; a straight thread does not seal at all and needs a separate sealing element.
| System | Seal mechanism | Hydraulic rating |
|---|---|---|
| NPT | Taper + PTFE tape or anaerobic pipe-dope | Low–medium pressure, not recommended for high cycle |
| NPTF (Dryseal) | Taper + controlled crest/root deformation | Better than NPT, still not preferred for hi-pressure |
| BSPT (R/Rc) | Taper + sealant or bonded seal | Equivalent to NPT, non-US |
| BSPP (G) | Parallel — needs bonded washer, O-ring boss, or face seal | Excellent with O-ring boss |
| SAE J1926 / J514 ORB | Straight UN thread + O-ring on the boss face (Code 61/62 ports) | The modern hydraulic standard in North American mobile equipment |
| DIN 3852 (ED-seal) | Metric straight + elastomer bonded washer | European hydraulic standard |
| DIN 24960 / ISO 6149 | Metric straight + O-ring boss | International hydraulic, gaining adoption |
| JIC 37° flare (AN/MS) | Metal-to-metal flare seal, threads only retain | Aerospace and high-vibration hydraulics |
Rule: if the application is above ~20 MPa (3000 psi) or sees high cycle, specify ORB or face-seal — never NPT.
14. Designation strings — examples
1/4-20 UNC-2A LH External UNC, standard class, left-hand
1/4-20 UNC-2B Internal UNC, standard class
M10×1.5 - 6g External metric coarse, standard
M10×1.5 - 6H Internal metric coarse, standard
M10×1.25 - 6g External metric fine
M8×1×100/100/12 STI Tapped for Heli-Coil 1-D length insert
1/2-14 NPT 1/2 NPS tapered pipe, 14 TPI
1/2-14 BSP-Pf BSPP parallel internal female (older notation)
G 1/2 BSPP, ISO 228-1 (preferred notation)
Rc 1/2 BSPT internal taper, ISO 7-1
2-12 UN-3B-Stub-ACME Stub-ACME, 2" major, 12 TPI, close internal
Tr 40×7 - 7e Trapezoidal external, std fit
Tr 40×14(P7) - 7e Two-start, 14 lead, 7 pitch
S 40×7 DIN 513 sawtooth/buttress
1.5-4 ACME-2G Standard ACME, 1.5" × 4 TPI
#10-32 UNF-2A Numbered-gauge inch fine
UNJ 1/4-28-3A Aerospace UNJ controlled-root
MJ 8×1.25-4h6h Aerospace MJ metric
A complete drawing callout includes: size–pitch–series–class–hand–depth (and any special features like STI, dry-film lubricant, plating).
15. Selection heuristics
- General-purpose fastener (steel, aluminium, brass): UNC-2A or M coarse 6g. Default to coarse unless you have a reason.
- Thin-walled material, finer adjustment, vibration sensitivity: UNF, UNEF, or M fine pitch (M8×1, M10×1.25, M12×1.5).
- Soft / repair / repeat-assembly tapped hole: STI + Heli-Coil. M6×1 STI is the workhorse for instrument enclosures.
- High-cycle aerospace primary structure: UNJ or MJ. Often combined with thread-locking patch or Heli-Coil.
- Pipe + threaded joint + sealant acceptable: NPT (US) or BSPT/R (rest of world).
- High-pressure hydraulic, leak-free, vibration: SAE J1926 / J514 ORB (North America), ISO 6149 / DIN 24960 (rest of world), or JIC 37° flare for aerospace/mobile.
- Lead screw, manual jack, vise, press: ACME (inch) or Tr (metric). Centralizing ACME if side-load is significant.
- Lead screw, high-precision low-friction motion: not on this list — use ballscrew (not a thread family — rolling element). See
[[Engineering/Tier3/bearings-taxonomy]]. - One-direction high axial load (vise, press ram): Buttress (B7 or DIN 513). Load flank near-perpendicular to axis.
- Large clamping, low thread count, robust: 1.5”-6 ACME or M48×5 — common on jaw chucks and gate-valve stems.
- Microscope, watch, surgical implant: UNM 1.20×0.25, M1, M1.4, M1.6. Avoid BA in new design.
- Through-bolt heavy structural (bridge, crane): A325 / A490 (inch) or 8.8 / 10.9 / 12.9 (metric) — see
[[Engineering/fasteners-bolts]]for material; thread is UN or M coarse. - Plastic housing: Type 23/25 Plastite or thread-forming Taptite into a moulded boss. Engagement length = 2 × D minimum.
16. Cross-references
[[Engineering/fasteners-bolts]]— bolt grades, preload, torque–tension, locking.[[Engineering/Tier3/fasteners-taxonomy]]— full fastener family taxonomy.[[Engineering/Tier3/seals-taxonomy]]— O-rings, bonded seals, face seals, ED-seal.[[Engineering/Tier3/bearings-taxonomy]]— ballscrews and leadscrews (motion side).[[Engineering/Tier3/steel-grades]]— fastener-grade steels (1018, 1144, 4140, 4340).[[Engineering/Tier3/stainless-steels]]— A2-70 / A4-80 stainless fastener grades.
17. Citations
ASME / ANSI
- ASME B1.1 — Unified Inch Screw Threads (UN and UNR).
- ASME B1.3 — Screw Thread Gauging Systems for Acceptability.
- ASME B1.5 — ACME Screw Threads.
- ASME B1.8 — Stub-ACME Screw Threads.
- ASME B1.9 — Buttress Inch Screw Threads.
- ASME B1.10 — Unified Miniature Screw Threads.
- ASME B1.13M — Metric Screw Threads — M Profile.
- ASME B1.15 — Unified Inch Screw Threads (UNJ Profile).
- ASME B1.20.1 — Pipe Threads, General Purpose (Inch) — NPT.
- ASME B1.20.3 — Dryseal Pipe Threads (Inch) — NPTF.
- ASME B1.21M — Metric Screw Threads — MJ Profile.
- ANSI/ASME B18.6.4 — Thread-Forming and Thread-Cutting Tapping Screws.
ISO
- ISO 68-1 — Basic profile, metric screw threads.
- ISO 261 — General-purpose metric screw threads — General plan.
- ISO 262 — Selected sizes for screws, bolts, and nuts.
- ISO 965-1/2/3 — Tolerances for metric screw threads.
- ISO 1501 — ISO miniature screw threads.
- ISO 2902 — ISO trapezoidal screw threads — General plan.
- ISO 2903 — ISO trapezoidal screw threads — Tolerances.
- ISO 5408 — Screw threads — Vocabulary.
- ISO 5855-1/2/3 — Aerospace — MJ threads.
- ISO 7-1 — Pipe threads where pressure-tight joints are made on the threads (BSPT).
- ISO 228-1 — Pipe threads where pressure-tight joints are not made on the threads (BSPP).
- ISO 1179 — Connections for fluid power — Ports and stud ends with ISO 228-1 threads.
- ISO 6149 — Connections for fluid power — Ports and stud ends with ISO 261 metric threads and O-ring sealing.
DIN
- DIN 103 — Trapezoidal screw threads.
- DIN 405 — Round screw threads (Rd).
- DIN 513 — Buttress threads (Sägengewinde).
- DIN 3852 — Threaded connection ports — ED-seal.
- DIN 7500 — Thread-forming screws — metric.
- DIN 24960 — Hydraulic connections, metric, O-ring boss.
SAE
- SAE J514 — Hydraulic Tube Fittings.
- SAE J1926 — Connections for Fluid Power and General Use — Ports and Stud Ends with ISO 261 Threads and O-Ring Sealing.
MIL / NAS
- MIL-S-8879 (superseded) — Screw Threads, Controlled Radius Root with Increased Minor Diameter, General Specification for. Now ANSI/ASME B1.15.
- NAS 1351, NAS 6603, NAS 6606 — Aerospace bolt families using UNJ.
Industry references
- Industrial Fasteners Institute (IFI) Inch Fastener Standards — current edition.
- Machinery’s Handbook — thread tables, tap drill, geometry math.
- Heli-Coil Engineering Manual — STI tap-drill sizing and insert selection.