Unified Inch Threads (UNC/UNF) + Whitworth (BSW/BSF) Reference
Two inch-thread families, two thread angles—Whitworth is often the default in Taiwan unless UNC/UNF is specified
“Inch thread” is not a single system. Modern industrial drawings commonly use the Unified Thread Standard (UNC/UNF) with a 60° thread angle, while many legacy British applications use Whitworth-form threads (BSW/BSF) with a 55° angle and different crest/root geometry. Even when nominal size and TPI look similar, cross-system mixing can create poor flank contact, unstable load distribution, accelerated wear, or inconsistent gaging results. For inch-thread requests without a specified standard family, UJEN will typically propose Whitworth first because Whitworth sizes are more widely stocked and cost-effective in Taiwan supply chains; however, we also support UNC/UNF based on customer requirements. This page provides practical callout formats, quick tables (including sizes down to near 3 mm major diameter and up to 5/8), and a clear Whitworth section to keep drawings, purchasing, and inspection aligned.
What This Page Covers
This reference covers Unified inch threads (UNC/UNF) and Whitworth-form inch threads (BSW/BSF), including how to write callouts, quick size tables, and practical compatibility warnings.
Unified vs Whitworth: The Key Difference
- Unified (UNC/UNF): 60° thread angle, modern industrial standard family
- Whitworth (BSW/BSF): 55° thread angle, different crest/root geometry, legacy British standard family
Do not treat “same diameter + same TPI” as automatic interchangeability across these families. The thread form (angle and geometry) matters for flank contact and load distribution.
Applicable Standards (Unified UNC/UNF)
- ASME B1.1: Unified inch screw threads (thread form, series, class, designation)
- ASME B1.2: gages and gaging for Unified inch screw threads
- ASME B1.3: gaging systems and acceptance criteria (when a gaging system is used)
Recommended Callout Format (UNC/UNF)
Specify size – TPI – series – class of fit. Use A for external threads and B for internal threads.
- External: 1/4-20 UNC-2A
- Internal: 1/4-20 UNC-2B
- External: 5/16-24 UNF-2A
- Internal: 5/16-24 UNF-2B
- Left-hand thread (if applicable): add “LH” (example: 1/4-20 UNC-2A LH)
- Thread length / engagement: specify where functional
Unified Quick Reference (Down to Near 3 mm Major Dia, Up to 5/8)
This table supports fast drawing/RFQ alignment. Always state the series (UNC or UNF) and class (e.g., 2A/2B) when you need defined gaging and acceptance.
| Nominal Size | Major Dia (mm) | UNC (TPI) | UNF (TPI) | Typical Callout Example |
|---|---|---|---|---|
| #4 | 2.845 | 40 | 48 | #4-40 UNC-2A / #4-40 UNC-2B |
| #5 | 3.175 | 40 | 44 | #5-40 UNC-2A / #5-40 UNC-2B |
| #6 | 3.505 | 32 | 40 | #6-32 UNC-2A / #6-32 UNC-2B |
| #8 | 4.166 | 32 | 36 | #8-32 UNC-2A / #8-32 UNC-2B |
| #10 | 4.826 | 24 | 32 | #10-24 UNC-2A / #10-24 UNC-2B |
| #12 | 5.486 | 24 | 28 | #12-24 UNC-2A / #12-24 UNC-2B |
| 1/4 | 6.350 | 20 | 28 | 1/4-20 UNC-2A / 1/4-20 UNC-2B |
| 5/16 | 7.938 | 18 | 24 | 5/16-18 UNC-2A / 5/16-18 UNC-2B |
| 3/8 | 9.525 | 16 | 24 | 3/8-16 UNC-2A / 3/8-16 UNC-2B |
| 7/16 | 11.112 | 14 | 20 | 7/16-14 UNC-2A / 7/16-14 UNC-2B |
| 1/2 | 12.700 | 13 | 20 | 1/2-13 UNC-2A / 1/2-13 UNC-2B |
| 5/8 | 15.875 | 11 | 18 | 5/8-11 UNC-2A / 5/8-11 UNC-2B |
Whitworth Threads (BSW/BSF): Separate Family
Whitworth-form threads use a 55° thread angle and different crest/root geometry than Unified threads. Always specify the series explicitly as BSW or BSF.
Whitworth Quick Reference (BSW/BSF Common Sizes)
This is a compact reference for common BSW and BSF thread counts. For purchasing and inspection, specify the series (BSW or BSF) and do not substitute UNC/UNF unless the design explicitly allows it.
| Nominal Size | BSW (TPI) | BSF (TPI) | Notes |
|---|---|---|---|
| 1/8 | 40 | - | BSW commonly used at this size |
| 3/16 | 24 | 32 | Different TPI between BSW and BSF |
| 1/4 | 20 | 26 | Same TPI as 1/4-20 UNC but different 55° form |
| 5/16 | 18 | 22 | Different system and form |
| 3/8 | 16 | 20 | Different system and form |
| 7/16 | 14 | 18 | Different system and form |
| 1/2 | 12 | 16 | Different system and form |
| 5/8 | 11 | 14 | Different system and form |
Interchangeability Warning
Same nominal size and TPI does not guarantee interchangeability across families. Example: 1/4 BSW (20 TPI) and 1/4-20 UNC (20 TPI) may start engagement, but 55° vs 60° flank angles change contact patterns. This can lead to unstable load distribution, accelerated wear, and inconsistent gaging results. Specify the standard family (Unified or Whitworth), not only size and TPI.
Measurement Basis Note (Pre- vs Post-Plating)
If parts are plated, define whether acceptance is based on pre-plating gauging or post-plating gauging. Post-plating gage-pass requirements may require a different process route and quotation basis.
UJEN Practice (Whitworth Priority in Taiwan, UNC/UNF Available)
For inch-thread requests without a specified standard family, UJEN will typically propose Whitworth threads (BSW/BSF) first, because Whitworth sizes are more widely stocked and cost-effective in Taiwan supply chains. When your project requires Unified standards, we also support UNC/UNF and can supply according to the specified series and class (e.g., UNC-2A/2B, UNF-2A/2B).
To avoid interchangeability risks, please specify the thread family explicitly on the drawing/RFQ: Unified (UNC/UNF) or Whitworth (BSW/BSF). If parts are plated, also define the measurement basis (pre- or post-plating gauging).
RFQ Checklist (Copy/Paste Ready)
- Thread family: Whitworth (BSW/BSF) or Unified (UNC/UNF)
- If not specified: UJEN typically proposes Whitworth first (Taiwan supply chain availability and cost advantage)
- Internal or external thread
- Callout:
- Unified: size – TPI – series – class (e.g., 1/4-20 UNC-2A / 1/4-20 UNC-2B)
- Whitworth: size – series (BSW or BSF) – TPI if needed (e.g., 1/4 BSW or 1/4 BSF)
- Left-hand thread (if applicable): add LH
- Plating/coating requirement and measurement basis (pre- vs post-plating gauging)