Reference Pages - Threads

 There are a lot of peculiar threads which are a bit of a puzzle to anyone new to vintage bikes. One of the challenges is that the bikes we are interested in, and perhaps for some of us this is why we are interested in them, come from a time when automotive engineering was developing at pace. For those of us interested in a world prior to the Japanese domination of the industry, thse words and numbers may be useful.

The following pages provide descriptions of the threads in common use; the drill sizes, pitches and hex sizes associated with them, and a few interesting tit bits of information...

Data Tables for Threads in common use

The following table offers some guidance on the use of the various thread forms. You can use it to give yourself some clues as to the thread form you are looking at, depending upon what it is used for.

Form	Common Usage
BA	British Association – a 47.5° thread form typically used for Lucas & Wipac electrical equipment
BSC, BSCy, CEI	British Standard Cycle or Cycle Engineers Institute. A 60° thread form with a fine pitch generally used for most cycle parts – wheel spindles, steering column, mudguard stay bolts. The CEI thread series was superseded by BSC, which are the same thread profiles; the BSC series dropped some of the less popular CEI sizes.
BSW	British Standard Whitworth. A 55° thread form with a coarse pitch generally used for threads into alloy – engine side cover screws, crankcase screws, and cylinder head studs (crankcase end).
BSF	British Standard Fine. A 55° thread form with a fine pitch generally used for cycle parts or threads requiring a higher degree of resistance to loosening – engine bolts, cylinder head studs (head end).
BSP	British Standard Pipe. Another Whitworth-based 55° thread form used for threads on petrol taps and similar products. Pipe threads can be applied to parallel or tapered parts – tapered pipe threads (BSPT) seal against themselves using PTFE tape; parallel pipe threads (BSPP) use fibre or other sealing washers.
UNF	Similar service to BSF but with a 60° thread form, appearing late in the 1960’s as the Unified series was adopted across the motor industry. Not used on Bantams until the B175 appeared.
UNC	Similar service to BSW but with a 60° thread form, appearing late in the 1960’s as the Unified series was adopted across the motor industry. Not used on Bantams until the B175 appeared.
Metric	Everything Japanese & European! But more curiously, the threads on your Jaeger derived Smiths Chronometric speedometer are metric. A 60° thread form.

Shrunken Heads

Sometimes life is made exciting for us techno-geeks by the illogical imagination of our forefathers, and there is no better example than a discussion about the size of hexagon heads applied to fasteners.

As we know, Whitworth spanner markings refer to the bolt diameter rather than the distance across the flats of the hexagon (A/F) as is common metric & Unified standards. You sometimes find imperial spanners marked, at one end, with two different BSF and BSW sizes which arises because as the lighter thread form BSF hexagon sizes can be one size smaller than the Whitworth hexagon for the same thread size, for example, the instance where a spanner (wrench) marked 7/16BSF is the same size as one marked 3/8BSW. In both cases the spanner jaw width of 0.710 in, the width across the hexagon flat, is the same.

This was the case prior to World War 2 and is the case for pre-war spanners which often turn up in your Granddad’s shed or in boxes of rusty old tools at car boot sales.

However, in World War II the size of the Whitworth hexagon was reduced to the same size as the equivalent BSF hexagon purely to save metal during the war, and this remained the case after the cessation of hostilities. Thus, it is today uncommon to encounter a Whitworth hexagon which takes the nominally correct spanner. Spanners in this case may be marked 7/16BS to indicate that they have a jaw size of 0.710 in and are designed to take either the (later) 7/16 BSW or 7/16 BSF hexagon.

Thread Data

The following tables show the hexagon sizes, pitch, tapping and clearances sizes for various threads where they are available. For the uninitiated, the tapping size is the size of the hole you have to drill to cut a female thread; the clearance size is the drill which will allow a male thread to pass through.

You will see some of the tapping and clearance holes are presented as rather esoteric sizes. Many of these threads use Letter or Number drills to obtain the correct tapping and clearance sizes.

You should be aware that some series share some sizes & pitches with others, notably BSW & UNC (look at the 7/16” threads) and BSC & BSF (look at the ¼” threads) but are different thread forms. You will find that if you screw a BSW bolt into a UNC nut it will be loose; screwing a BSC bolt into a BSF nut will be stiff.

BSW Thread Data

Thread Size BSW	Actual Hex Size mm	Actual Hex Size  Inches	Pitch	Tapping Size	Clearance size
1/8	8.64	0.340	40 tpi	2.55 mm	3.3 mm
3/16	11.3	0.445	24 tpi	3.7 mm	5 mm
1/4	13.34	0.525	20 tpi	5 mm	6.5 mm
5/16	15.24	0.6	18 tpi	6.4 mm	8 mm
3/8	18.03	0.710	16 tpi	7.9 mm	9.8 mm
7/16	20.83	0.82	14 tpi	9.2 mm	29/64”
1/2	23.37	0.92	12 tpi	10.4 mm	33/64“
9/16	25.65	1.010	12 tpi	12 mm	37/64”
5/8	27.94	1.100	11 tpi	17/32”	41/64”
11/16	30.48	1.200	11 tpi	15 mm
3/4	33.02	1.300	10 tpi	16.25 mm	49/64”
7/8	37.59	1.480	9 tpi	3/4”	57/64”
1	42.42	1.670	8 tpi	55/64”	1 1/64”

BSF Thread Data

Thread Size BSF	Actual Hex Size mm	Actual Hex Size  Inches	Pitch	Tapping Size	Clearance size
3/16	11.3	0.445	32 tpi	3.95 mm	4.9 mm
d1/4	13.34	0.525	26 tpi	5.25 mm	6.5 mm
5/16	15.24	0.6	22 tpi	6.75 mm	8 mm
3/8	18.03	0.710	20 tpi	8.25 mm	9.8 mm
7/16	20.83	0.82	18 tpi	9.7 mm	29/64”
1/2	23.37	0.92	16 tpi	7/16”	33/64“
9/16	25.65	1.010	16 tpi	½”	37/64”
5/8	27.94	1.100	14 tpi	14 mm	41/64”
11/16	30.48	1.200	12 tpi	39/64”	45/64”
3/4	33.02	1.300	12 tpi	16.75 mm	49/64”
7/8	37.59	1.480	11 tpi	25/32”	57/64”
1	42.42	1.670	10 tpi	57/64”	1 1/64”

BSC Thread Data

Thread Size BSC	Actual Hex Size mm	Actual Hex Size  Inches	Pitch	Tapping Size
1/8	8.64	0.340	40 tpi	0.0995”
3/16	11.3	0.445	32 tpi	0.1562”
1/4	13.34	0.525	26 tpi	0.2130”
5/16	15.24	0.6	26 tpi	0.2770”
3/8	18.03	0.710	26 tpi	0.339”
7/16	20.83	0.82	20/26 tpi	X, W
1/2	23.37	0.92	20/26 tpi	11.7 mm, 11.5 mm
9/16	25.65	1.010	20/26 tpi	13.3 mm, 13 mm
5/8	27.94*	1.100	20/26 tpi	15 mm, 14.6 mm
11/16	30.48	1.200	20/26 tpi	16.5 mm, 16.2 mm
3/4	33.02*	1.300	20/26 tpi	23/32”, 45/64”
7/8	37.59	1.480	24 tpi	27/32”
1	42.42	1.670	26 tpi	31/32”

BA Thread Data

BA Thread Size	Spanner size  across flats		Thread Pitch	TPI	Thread OD	Tapping Size	Clearance size
10	0.117”	2.97 mm	0.0138”	72.6	0.067”	#56	#50
9	0.131”	3.33 mm	0.0154”	65.1	0.075”	#53	#48
8	0.152”	3.86 mm	0.0169”	59.1	0.086”	#51	#43
7	0.172”	4.37 mm	0.0189”	52.9	0.0982”	#48	#39
6	0.193”	4.90 mm	0.0209”	47.9	0.11”	#44	#33
5	0.220”	5.59 mm	0.0232”	43	0.126”	#40	#30
4	0.248”	6.30 mm	0.0260”	38.5	0.1420”	#34	#27
3	0.282”	7.16 mm	0.0287”	34.8	0.1610”	#30	#19
2	0.324”	8.23 mm	0.0319”	31.4	0.1850”	#26	#12
1	0.365”	9.27 mm	0.0354”	28.2	0.2090”	#19	#3
0	0.413”	10.49 mm	0.0394”	25.4	0.2360”	#10	B

Unified Thread Forms

Ariel began use of unified series threads with the Leader in 1959. You know that UNF & UNC, similar to metric series, use hex dimensions where the nominal spanner or socket size is the hexagon measured across the flats.

UNC Thread Data

Thread Size UNC	Actual Hex Size mm	Actual Hex Size  Inches	Pitch	Tapping Size	Clearance size
1/8 (5-40)	8.64	0.340	40 tpi	#38	#29, #30
3/16 (10-24)	11.3	0.445	24 tpi	#25	#7, #9
1/4 (1/4-20)	13.34	0.525	20 tpi	#7	F, H
5/16 (5/16-18)	15.24	0.6	18 tpi	F	P, Q
3/8 (3/8-16)	18.03	0.710	16 tpi	5/16”	W, X
7/16 (7/16-14)	20.83	0.82	14 tpi	U	29/64”, 15/32”
1/2 (1/2-13)	23.37	0.92	13 tpi	27/64”	33/64”, 17/32”
9/16 (9/16-12)	25.65	1.010	12 tpi	31/64”	37/64”, 19/32”
5/8 (5/8-11)	27.94*	1.100	11 tpi	17/32”	41/64”, 21/32”
3/4 (3/4-10)	33.02*	1.300	10 tpi	21/32”	49/64”, 25/32”
7/8 (7/8-9)	37.59	1.480	9 tpi	49/64”	57/64”, 29/32”
1 (1-8)	42.42	1.670	8 tpi	7/8”	1 1/64”, 1 1/32”

UNF Thread Data

Thread Size UNF	Actual Hex Size mm	Actual Hex Size  Inches	Pitch	Tapping Size	Clearance size
1/8 (5-44)	8.64	0.340	44 tpi	#37	#29, #30
3/16 (10-32)	11.3	0.445	32 tpi	#21	#7, #9
1/4 (1/4-28)	13.34	0.525	28 tpi	#3	F, H
5/16 (5/16-24)	15.24	0.6	24 tpi	I	P, Q
3/8 (3/8-24)	18.03	0.710	24 tpi	Q	W, X
7/16 (7/16-20)	20.83	0.82	20 tpi	25/64”	29/64”, 15/32”
1/2 (1/2-20)	23.37	0.92	20 tpi	29/64”	33/64”, 17/32”
9/16 (9/16-18)	25.65	1.010	18 tpi	33/64”	37/64”, 19/32”
5/8 (5/8-18)	27.94*	1.100	18 tpi	37/64”	41/64”, 21/32”
3/4 (3/4-16)	33.02*	1.300	16 tpi	11/16”	49/64”, 25/32”
7/8 (7/8-14)	37.59	1.480	14 tpi	13/16”	57/64”, 29/32”
1 (1-14)	42.42	1.670	14 tpi	15/16”	1 1/64”, 1 1/32”

British Standard Pipe Threads

BSP parallel & taper threads, used on petrol taps are British Standard Pipe or BSP. Bantams commonly use a 3/8" BSP thread in the tank and a 3/16” BSP thread on the tap outlet. BSP thread sizes are confusing because the size relates to the bore of the pipe, not to the actual thread size. Nominal thread diameters are useful for checking thread sizes and are shown below.

BSP Thread Data

Nominal Size	Male OD		Female ID		Pitch	Tapping Size	Clearance size
1/8 BSP	0.375"	9.5 mm	0.350"	8.9 mm	28 TPI	11/32”	W drill
3/16 BSP	0.425”	10.8 mm	0.375"	9.5 mm	19 TPI	13/32”	15/32”
1/4 BSP	0.515"	13.08 mm	0.468"	11.89 mm	19 TPI	15/32”	17/32”
3/8 BSP	0.650"	16.5 mm	0.609"	15.47 mm	19 TPI	19/32”	43/64”
1/2 BSP	0.825”	20.9 mm	0.73”	18.6 mm	14 TPI	3/4”	53/64”