Thread Standards Reference Guide
Complete charts for metric (M), unified (UNC/UNF), BSP (G) and NPT thread standards — pitch, TPI, tap drill sizes, minor diameters and tolerances for CNC machined parts.
Thread System Overview
| Standard | Name | Unit | Pitch Defined By | Thread Angle | Primary Regions | Use Cases |
|---|---|---|---|---|---|---|
| ISO Metric (M) | Metric Coarse/Fine | mm | Pitch (mm) | 60° | Global | General engineering, CNC parts |
| UNC | Unified National Coarse | inch | TPI (threads/inch) | 60° | USA, Canada | Fasteners, general assembly |
| UNF | Unified National Fine | inch | TPI | 60° | USA, Canada | Thin-wall, higher strength joints |
| BSP / G (BSPP) | British Standard Pipe Parallel | inch | TPI | 55° | UK, Europe, Asia | Hydraulic/pneumatic fittings |
| NPT | National Pipe Taper | inch | TPI | 60° | USA, Canada | Pipe fittings, pressure connections |
Threads Machined at Survi
We produce all five thread families in this guide — metric, UNC/UNF, BSP and NPT — using rigid tapping on our machining centres, single-point threading on our CNC lathes, and thread milling for larger or non-standard sizes. Thread quality is verified with Go/No-Go gauges as a minimum; for critical joints we additionally verify with full thread-form gauging.
Thread length, depth and runout are specified on every job card. For blind holes, we drill to allow a minimum of 1.5× diameter of full threads below the surface — a DFM practice that prevents tap breakage and ensures assembly margin. Let our engineering team DFM-review your threaded features before quoting.
Thread Charts
ISO metric coarse thread series (most common). All dimensions in mm.
| Designation | Nom. Ø (mm) | Pitch (mm) | Pitch Ø (mm) | Minor Ø (mm) | Tap Drill (mm) | Tolerance Class |
|---|---|---|---|---|---|---|
| M2 | 2.0 | 0.40 | 1.740 | 1.509 | 1.60 | 6H / 6g |
| M2.5 | 2.5 | 0.45 | 2.208 | 1.948 | 2.05 | 6H / 6g |
| M3 | 3.0 | 0.50 | 2.675 | 2.387 | 2.50 | 6H / 6g |
| M4 | 4.0 | 0.70 | 3.545 | 3.141 | 3.30 | 6H / 6g |
| M5 | 5.0 | 0.80 | 4.480 | 4.019 | 4.20 | 6H / 6g |
| M6 | 6.0 | 1.00 | 5.350 | 4.773 | 5.00 | 6H / 6g |
| M8 | 8.0 | 1.25 | 7.188 | 6.466 | 6.80 | 6H / 6g |
| M10 | 10.0 | 1.50 | 9.026 | 8.160 | 8.50 | 6H / 6g |
| M12 | 12.0 | 1.75 | 10.863 | 9.853 | 10.20 | 6H / 6g |
| M14 | 14.0 | 2.00 | 12.701 | 11.546 | 12.00 | 6H / 6g |
| M16 | 16.0 | 2.00 | 14.701 | 13.546 | 14.00 | 6H / 6g |
| M20 | 20.0 | 2.50 | 18.376 | 16.933 | 17.50 | 6H / 6g |
| M24 | 24.0 | 3.00 | 22.051 | 20.319 | 21.00 | 6H / 6g |
| M30 | 30.0 | 3.50 | 27.727 | 25.706 | 26.50 | 6H / 6g |
| M36 | 36.0 | 4.00 | 33.402 | 31.093 | 32.00 | 6H / 6g |
| M42 | 42.0 | 4.50 | 39.077 | 36.479 | 37.50 | 6H / 6g |
Unified National Coarse thread series. All dimensions in inches unless noted.
| Designation | Nom. Ø (in) | TPI | Pitch (mm) | Pitch Ø (in) | Minor Ø (in) | Tap Drill (in) | Tap Drill (mm) |
|---|---|---|---|---|---|---|---|
| #2-56 UNC | 0.0860 | 56 | 0.454 | 0.0744 | 0.0628 | #50 | 1.78 |
| #4-40 UNC | 0.1120 | 40 | 0.635 | 0.0958 | 0.0795 | #43 | 2.26 |
| #6-32 UNC | 0.1380 | 32 | 0.794 | 0.1177 | 0.0974 | #36 | 2.77 |
| #8-32 UNC | 0.1640 | 32 | 0.794 | 0.1437 | 0.1234 | #29 | 3.45 |
| #10-24 UNC | 0.1900 | 24 | 1.058 | 0.1629 | 0.1359 | #25 | 3.80 |
| 1/4-20 UNC | 0.2500 | 20 | 1.270 | 0.2175 | 0.1850 | #7 | 5.10 |
| 5/16-18 UNC | 0.3125 | 18 | 1.411 | 0.2764 | 0.2403 | F | 6.60 |
| 3/8-16 UNC | 0.3750 | 16 | 1.588 | 0.3344 | 0.2938 | 5/16 | 7.94 |
| 7/16-14 UNC | 0.4375 | 14 | 1.814 | 0.3911 | 0.3447 | U | 9.40 |
| 1/2-13 UNC | 0.5000 | 13 | 1.954 | 0.4500 | 0.3997 | 27/64 | 10.72 |
| 5/8-11 UNC | 0.6250 | 11 | 2.309 | 0.5660 | 0.5069 | 17/32 | 13.49 |
| 3/4-10 UNC | 0.7500 | 10 | 2.540 | 0.6850 | 0.6201 | 21/32 | 16.67 |
| 1-8 UNC | 1.0000 | 8 | 3.175 | 0.9188 | 0.8376 | 7/8 | 22.23 |
Unified National Fine — finer pitch than UNC, used where greater pullout strength or thin walls require more threads per unit length. Dimensions in inches.
| Designation | Nom. Ø (in) | TPI | Pitch Ø (in) | Minor Ø (in) | Tap Drill (in) | Tap Drill (mm) |
|---|---|---|---|---|---|---|
| #0-80 UNF | 0.0600 | 80 | 0.0519 | 0.0438 | 3/64 | 1.19 |
| #2-64 UNF | 0.0860 | 64 | 0.0759 | 0.0659 | #50 | 1.78 |
| #4-48 UNF | 0.1120 | 48 | 0.0985 | 0.0849 | #42 | 2.37 |
| #6-40 UNF | 0.1380 | 40 | 0.1218 | 0.1055 | #33 | 2.87 |
| #8-36 UNF | 0.1640 | 36 | 0.1460 | 0.1279 | #29 | 3.45 |
| #10-32 UNF | 0.1900 | 32 | 0.1697 | 0.1494 | #21 | 4.06 |
| 1/4-28 UNF | 0.2500 | 28 | 0.2268 | 0.2036 | #3 | 5.56 |
| 5/16-24 UNF | 0.3125 | 24 | 0.2854 | 0.2584 | I | 6.35 |
| 3/8-24 UNF | 0.3750 | 24 | 0.3479 | 0.3209 | Q | 8.33 |
| 1/2-20 UNF | 0.5000 | 20 | 0.4675 | 0.4350 | 29/64 | 11.51 |
| 3/4-16 UNF | 0.7500 | 16 | 0.7094 | 0.6688 | 11/16 | 17.46 |
| 1-12 UNF | 1.0000 | 12 | 0.9459 | 0.8918 | 59/64 | 23.42 |
BSPP (British Standard Pipe Parallel — G thread) — widely used in Europe, UK, India, China and Australia for hydraulic and pneumatic fittings. 55° thread angle, sealing via bonded seal or O-ring face seal.
| G Designation | Nom. Pipe Size (in) | TPI | Pitch (mm) | OD (mm) | Pitch Ø (mm) | Tap Drill (mm) |
|---|---|---|---|---|---|---|
| G 1/8 | 1/8" | 28 | 0.907 | 9.728 | 9.147 | 8.80 |
| G 1/4 | 1/4" | 19 | 1.337 | 13.157 | 12.301 | 11.80 |
| G 3/8 | 3/8" | 19 | 1.337 | 16.662 | 15.806 | 15.25 |
| G 1/2 | 1/2" | 14 | 1.814 | 20.955 | 19.793 | 18.60 |
| G 3/4 | 3/4" | 14 | 1.814 | 26.441 | 25.279 | 24.00 |
| G 1 | 1" | 11 | 2.309 | 33.249 | 31.770 | 30.50 |
| G 1¼ | 1¼" | 11 | 2.309 | 41.910 | 40.431 | 39.00 |
| G 1½ | 1½" | 11 | 2.309 | 47.803 | 46.324 | 45.00 |
| G 2 | 2" | 11 | 2.309 | 59.614 | 58.135 | 57.00 |
NPT (National Pipe Taper) — US and Canada standard for pipe fittings. Tapered 1° 47' (1/16 per foot) — seals by thread engagement with PTFE tape or pipe sealant.
| NPT Size | Nom. Pipe (in) | TPI | OD Start (mm) | Taper | Tap Drill (mm) | Engagement Length (mm) |
|---|---|---|---|---|---|---|
| 1/16 NPT | 1/16" | 27 | 7.895 | 1:16 | 6.50 | 6.5 |
| 1/8 NPT | 1/8" | 27 | 10.287 | 1:16 | 8.60 | 6.8 |
| 1/4 NPT | 1/4" | 18 | 13.716 | 1:16 | 11.50 | 9.5 |
| 3/8 NPT | 3/8" | 18 | 17.145 | 1:16 | 14.75 | 10.1 |
| 1/2 NPT | 1/2" | 14 | 21.336 | 1:16 | 18.00 | 13.6 |
| 3/4 NPT | 3/4" | 14 | 26.670 | 1:16 | 23.25 | 13.9 |
| 1 NPT | 1" | 11.5 | 33.401 | 1:16 | 29.50 | 17.3 |
| 1¼ NPT | 1¼" | 11.5 | 42.164 | 1:16 | 38.00 | 18.0 |
| 1½ NPT | 1½" | 11.5 | 48.260 | 1:16 | 44.00 | 18.4 |
| 2 NPT | 2" | 11.5 | 60.325 | 1:16 | 56.50 | 19.1 |
Choosing the Right Thread Standard for Your Application
The most common mistake in cross-border manufacturing is mixing thread standards. A US engineer specifying UNC fasteners assumes the supplier knows to use UNC taps — but without explicit callouts, a supplier in India or Europe may default to the nearest metric equivalent, resulting in an assembly that won't fit. Always specify the full thread callout (e.g., "M10×1.5 — 6H" or "3/8-16 UNC 2B") on your drawing, not just the nominal diameter.
For hydraulic and pneumatic systems, pay close attention to the BSP vs NPT distinction: BSP (G) threads are parallel with a flat-face seal; NPT threads are tapered and rely on thread interference for sealing. They use different sealants and cannot be interchanged even when the nominal sizes look similar on a chart.
How to Specify Threads on Your Drawing
Correct thread specification on your drawing avoids manufacturing errors. Use these formats:
| Thread Type | Drawing Callout | Meaning |
|---|---|---|
| Metric Coarse | M8 — 6H (internal) / M8 — 6g (external) | 8mm nominal, standard tolerance class |
| Metric Fine | M8×1.0 — 6H | 8mm nominal, 1.0mm pitch (fine) |
| UNC | 1/4-20 UNC-2B (internal) / UNC-2A (external) | 1/4" diameter, 20 TPI, Class 2 fit |
| UNF | 1/4-28 UNF-2B | 1/4" diameter, 28 TPI fine, Class 2 |
| BSP | G 1/4 A (external) / G 1/4 B (internal) | 1/4" BSP parallel |
| NPT | 1/4 NPT | 1/4" national pipe taper |
Need Threaded CNC Parts?
We machine all thread types — metric, UNC, BSP, NPT — in aluminium, steel, titanium and brass. Upload your drawing and specify your thread requirement for an instant quote.
Frequently Asked Questions
We understand that choosing a manufacturing partner requires clarity and confidence. Our FAQs address common queries about pricing, quality, delivery timelines, quotations, and service support. Whether you're exploring our cost-effective solutions or need details on our processes, you'll find the answers here.
How does Survi offer pricing that is 75% lower than the U.S. and Chinese market?
At Survi, we optimize costs through efficient manufacturing processes, strategic sourcing, and lean operations. By leveraging advanced automation and minimizing overhead, we provide high-quality parts at a fraction of the typical market price—without compromising on precision or durability.
How does Survi ensure high-quality parts?
Quality is at the core of everything we do. We use cutting-edge technology, rigorous quality control processes, and skilled craftsmanship to meet and exceed industry standards. Every part undergoes thorough inspection to ensure precision, durability, and compliance with your specifications.
What is your typical lead time for orders?
Lead times vary based on the complexity and volume of the order, but we prioritize timely delivery. Most projects are completed within a few days to a couple of weeks. We work closely with our clients to ensure deadlines are met without compromising quality.
How quickly can I get a quotation?
We provide fast and accurate quotations, typically within 24 hours of receiving your request. For complex projects, we may require additional details, but we always strive for a quick turnaround.
What support do you offer after an order is placed?
We provide comprehensive service support before, during, and after production. Our team is available to address any concerns, assist with technical queries, and ensure a seamless experience from order placement to final delivery.
Which industries do you serve?
We cater to a wide range of industries, including automotive, aerospace, medical, robotics, and consumer electronics. Whether you need prototyping or high-volume production, we have the expertise to deliver top-notch solutions.
Can you handle custom manufacturing requests?
Absolutely! We specialize in custom machining, sheet metal fabrication, 3D printing, injection molding, and more. Share your design requirements with us, and we'll create a tailored solution to meet your needs.