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Resources/Threads/BSW / BSF

Threads

BSW and BSF Whitworth threads: sizes and threads per inch

BSW (British Standard Whitworth) and BSF (British Standard Fine) are 55° inch threads with rounded crests and roots, defined in the UK by BS 84. Largely replaced by metric and Unified threads, they remain in use for older machinery, some construction hardware and trade fasteners in India, the UK and other Commonwealth markets.

Names by standard

Standards in each region

RegionSystemDesignationStandardMatch
USANo US standard1/2 in BSWWhitworth threads are not standardised in the USA; the British standard is used where they are needed.BS 84Same material
UKBS Whitworth1/2 in BSW, 1/2 in BSFGoverning standard; covers BSW from 1/8 in to 6 in, BSF and related Whitworth-form series. BSW dates from 1841, BSF from 1908.BS 84:2007Same material
EUDIN (legacy)W 1/2"Former German Whitworth thread standard, superseded by metric threads. No current EN or ISO standard covers BSW.DIN 11 (withdrawn)Same material
IndiaTrade usage (BS 84 sizes)1/2" BSWNo current Indian Standard for Whitworth threads was identified. IS 4218 metric threads are standard for new fasteners; BSW sizes persist in trade, construction hardware and older equipment.BS 84Same material
JapanFormer JIS WhitworthW1/2Whitworth threads were removed from JIS but W-marked parts are still sold for construction, electrical and plumbing hardware.Withdrawn from JIS (reported 1968)Same material

Thread form

Geometria

Thread angle
55°, symmetric (27.5° flanks)
Crests and roots
Rounded, radius about 0.137 × pitch
Thread depth
About 0.640 × pitch
Pitch expressed as
Threads per inch (TPI)
Designation
Diameter + series, e.g. 3/8 in BSW or 3/8 in BSF; Japan uses W3/8
Related form
BSP pipe threads use the same 55° Whitworth profile on different diameters

Size chart

Common sizes

DimensioneMajor diameter (in)Major diameter (mm)BSW TPIBSF TPI
1/4 in0.2506.3502026
5/16 in0.31257.9381822
3/8 in0.3759.5251620
7/16 in0.437511.1131418
1/2 in0.50012.7001216
9/16 in0.562514.2881216
5/8 in0.62515.8751114
3/4 in0.75019.0501012
7/8 in0.87522.225911
1 in1.00025.400810

BSW continues above 1 in (to 6 in) and below 1/4 in (from 1/8 in). Pitch in mm = 25.4 ÷ TPI, e.g. 1/2 in BSW = 2.117 mm.

In practice

Uses and compatibility

Where it is used

  • Replacement fasteners for older British, Indian and Commonwealth machinery
  • Construction and fabrication hardware sold in inch sizes in India
  • Tripod and camera mounts (1/4 in-20 and 3/8 in-16 Whitworth)
  • Vintage vehicle and machine restoration

What interchanges

  • BSW and BSF of the same diameter do not mate; the pitches differ at every size in the table.
  • BSW and UNC share TPI at several sizes (1/4, 5/16, 3/8, 7/16, 9/16, 5/8, 3/4, 7/8 and 1 in) but not at 1/2 in (12 vs 13). The 55° and 60° forms do not seat correctly, so substitution reduces strength and is not recommended.
  • BSF does not match UNF at the common sizes (e.g. 1/4 in BSF 26 TPI vs UNF 28 TPI).
  • BSW and BSP share the 55° form, but a 1/4 BSP pipe thread is about 13.2 mm in diameter, not 1/4 in. They are different series.
  • Whitworth threads do not interchange with metric threads.

Domande e risposte

Common questions

What is the thread angle of BSW?

BSW and BSF use a 55° thread angle with rounded crests and roots. Unified and metric threads use 60°.

Is 1/2 BSW the same as 1/2 UNC?

No. 1/2 in BSW has 12 threads per inch and 1/2 in UNC has 13, and the thread angles differ (55° vs 60°).

Is BSW still used in India?

Yes, in trade and older equipment. New Indian fastener standards use IS 4218 metric threads, but BSW sizes remain common for construction hardware, replacements and some general fasteners.

What is the difference between BSW and BSF?

Both use the 55° Whitworth form. BSF has more threads per inch and a shallower thread for the same diameter, for example 3/8 in BSF has 20 TPI against 16 for BSW.

From the Corona catalog

Products listed with BSW / BSF

Barre e aste

Barre filettate

Diametro: 1/4-2 1/2 in / M6-M60 · Lunghezza: 3-20 ft / 1-6 m

Specifiche tecniche
Barre e aste

Tiranti (filettati alle due estremità)

Diametro: 1/2-2 1/2 in / M12-M60 · Lunghezza: 3-20 ft / 1-6 m

Specifiche tecniche
Componenti di supporto per tubazioni

Aste / barre filettate

Diametro: 1/4" - 2-1/2" / M6 - M60 · Lunghezza: 2 ft - 19 ft (comuni 3, 6, 10, 12 ft) / 1 m - 5,8 m (comuni 1, 2, 3 m)

Specifiche tecniche
Invia una richiesta

Keep reading

UNC / UNFUNC (Unified coarse) and UNF (Unified fine) are 60° inch threads designated by diameter and threads per inch, such as 1/2-13 UNC or 1/2-20 UNF.ISO metric (M)ISO metric threads are 60° V-threads designated by M, the major diameter in millimetres and, for fine pitches, the pitch in millimetres (M12 is coarse, M12×1.25 is fine).BSP vs NPTBSP pipe threads use the 55° Whitworth form, either parallel (G, ISO 228-1) or taper (R, Rp, Rc, ISO 7-1 and EN 10226), while NPT uses a 60° form tapered 1 in 16 to ASME B1.20.1.Rolled vs cut threadsA rolled thread is cold formed by pressing hardened dies into a blank, while a cut thread is machined by removing metal.

References

Sources

  1. British Standard Whitworth (Wikipedia) — BSW TPI, major diameters, 55° profile, radius and depth factors.
  2. British Standard Fine (Wikipedia) — BSF TPI and 1908 introduction.
  3. BSI: BS 84:2007 Parallel screw threads of Whitworth form — Scope and current status.
  4. MISUMI: Whitworth screws (withdrawn from JIS, still used in Japan)

Values summarize published limits and typical data. The governing standard and the current edition control; check them before specifying.