Question 1

What drill size do I need for a 1/8" BSP tap?

Accepted Answer

You need an 8.80 mm drill bit for a 1/8" BSP tap. The 1/8" BSP thread has 28 threads per inch (TPI) and an outside diameter of 9.73 mm. An 8.8 mm drill is the exact size — check your drill index as this size is commonly included in HSS sets.

Question 2

What drill size do I need for a 1/4" BSP tap?

Accepted Answer

You need an 11.80 mm drill bit for a 1/4" BSP tap. The 1/4" BSP thread has 19 TPI and an outside diameter of 13.16 mm. 11.8 mm gives approximately 75% thread engagement in steel, which is the industry standard. This size appears on pressure gauge ports, small hydraulic adaptors, and oil filler plugs on many tractors and implements.

Question 3

What drill size do I need for a 3/8" BSP tap?

Accepted Answer

You need a 15.00 mm drill bit for a 3/8" BSP tap. The 3/8" BSP thread has 19 TPI and an outside diameter of 16.66 mm. 15 mm is a standard drill size found in most workshop sets and gives correct thread engagement for steel and cast iron.

Question 4

What drill size do I need for a 1/2" BSP tap?

Accepted Answer

You need an 18.60 mm drill bit for a 1/2" BSP tap. The 1/2" BSP thread has 14 TPI and an outside diameter of 20.96 mm. The 1/2" BSP is the most common fitting size on UK farm machinery — found on tractor hydraulic systems, air lines, and water connections. An 18.5 mm drill is the nearest standard size and is acceptable for most applications in steel.

Question 5

What drill size do I need for a 3/4" BSP tap?

Accepted Answer

You need a 24.30 mm drill bit for a 3/4" BSP tap. The 3/4" BSP thread has 14 TPI and an outside diameter of 26.44 mm. A 24 mm drill is the nearest standard size and is widely used as a substitute in mild steel. 3/4" BSP is very common on tractor hydraulic systems, slurry equipment, and larger pipe connections.

Question 6

What drill size do I need for a 1" BSP tap?

Accepted Answer

You need a 30.50 mm drill bit for a 1" BSP tap. The 1" BSP thread has 11 TPI and an outside diameter of 33.25 mm. 30.5 mm is the exact tapping drill size — a 30 mm or 31 mm drill can be substituted in soft steel but 30.5 mm is correct per BS 21.

Question 7

What drill size do I need for a 1-1/4" BSP tap?

Accepted Answer

You need a 39.20 mm drill bit for a 1-1/4" BSP tap. The 1-1/4" BSP thread has 11 TPI and an outside diameter of 41.91 mm. This size is common on slurry tanker fittings and large bore water supply connections on farm buildings.

Question 8

What drill size do I need for a 1-1/2" BSP tap?

Accepted Answer

You need a 45.10 mm drill bit for a 1-1/2" BSP tap. The 1-1/2" BSP thread has 11 TPI and an outside diameter of 47.80 mm. A 45 mm drill is the nearest standard size. This size is used on large bore pipework for slurry, water supply mains, and pneumatic systems.

Question 9

What drill size do I need for a 2" BSP tap?

Accepted Answer

You need a 57.20 mm drill bit for a 2" BSP tap. The 2" BSP thread has 11 TPI and an outside diameter of 59.61 mm. At this size most tapping is done on a pillar drill or milling machine — hand tapping a 2" BSP is very difficult and risks a snapped tap. Allow plenty of cutting fluid.

Question 10

Why can't I calculate BSP tapping drill sizes from the nominal size?

Accepted Answer

BSP is a legacy designation where the nominal size (e.g. 1/2") originally referred to the bore of old iron water pipe, not any external or thread dimension. The actual thread outside diameter and pitch have no simple mathematical relationship to the nominal size — a 1/2" BSP has an outside diameter of almost 21 mm, not 12.7 mm. Always use a reference table — never try to derive BSP drill sizes by calculation.

Question 11

What is the difference between BSPP and BSPT? Do they need different tapping drill sizes?

Accepted Answer

BSPP (parallel, G thread) and BSPT (tapered, R thread) share the same thread form, TPI and pitch for each nominal size, so the tapping drill sizes are identical for both. The difference is in the taper: BSPT tapers at 1 in 16 to create a self-sealing metal-to-metal joint. BSPP is parallel and relies on a bonded seal, O-ring, or PTFE tape. BSPP is the most common form on modern UK farm machinery.

Question 12

What percentage thread engagement do these BSP drill sizes give?

Accepted Answer

The drill sizes in this calculator are sized for approximately 75% thread engagement, which is the standard recommendation for steel, cast iron, and aluminium. 75% gives full working strength — increasing thread engagement beyond 75% does not meaningfully improve pull-out strength but significantly increases the risk of tap breakage.

Question 13

Where do I find BSP threads on farm machinery?

Accepted Answer

BSP threads are used throughout UK farm machinery wherever pipes, hoses, and fittings connect. Common locations include: tractor hydraulic system ports and couplings (typically 1/4" to 3/4" BSP), pressure gauge and sensor ports (1/4" BSP most common), pneumatic air line connections (1/4" to 1/2" BSP), water bowser and tank fittings (3/4" to 2" BSP), slurry tanker connections and agitators (1-1/4" to 2" BSP), and fuel system fittings. When in doubt, measure the OD of the male fitting with a vernier caliper and compare against the OD column in the table above.

Question 14

How do I identify a BSP fitting if I don't know the size?

Accepted Answer

Measure the outside diameter of the male thread (or inside diameter of the female fitting) with a vernier caliper, then compare against the OD column in the table above. BSP outside diameters range from 9.73 mm (1/8") to 59.61 mm (2"). You can also count the threads per inch (TPI) using a thread gauge — 28 TPI is always 1/8" BSP, 19 TPI is 1/4" or 3/8", 14 TPI is 1/2" to 7/8", and 11 TPI is 1" and above.

Question 15

What cutting fluid should I use when tapping BSP threads?

Accepted Answer

For steel, use a dedicated tapping fluid such as Trefolex, Tap Magic, or a sulphurised cutting oil. These reduce friction and heat significantly, extending tap life and improving thread quality. For cast iron, dry tapping or compressed air to clear chips is often preferred — cutting oils can cause cast iron to gall. For aluminium, a light oil or WD-40 is acceptable, though dedicated aluminium cutting fluid gives better results. Never tap dry in steel — it is the most common cause of tap breakage.

BSP Size	Typical Location on Farm Machinery
1/8" BSP	Grease nipples, pressure switches, small sensor ports
1/4" BSP	Pressure gauges, hydraulic test points, oil plugs
3/8" BSP	Fuel connections, hydraulic block ports, coolant pipes
1/2" BSP	Tractor hydraulics, air lines, water connections — most common
3/4" BSP	Hydraulic supply lines, slurry agitators, water bowsers
1" BSP	Main hydraulic return lines, large bore pipe joints
1-1/4" BSP	Slurry tanker fittings, irrigation mains
1-1/2" BSP	Large bore slurry and water pipework
2" BSP	Tank fittings, bulk water connections, manifold ports

BSP Tap Drill Size Calculator

BSP Tapping Drill Size Lookup

BSP Tap Drill Size Chart — All Sizes 1/8″ to 2″

Where Do BSP Threads Appear on Farm Machinery?

BSPP vs BSPT — Which Do You Have and Which Tapping Drill Do You Need?

How to Tap a BSP Thread — Step by Step

Frequently Asked Questions — BSP Tap Drill Sizes

Related Tools

BSP Size	TPI	OD (mm)	Tapping Drill	Nearest Std Drill	Common Use
1/8" BSP	28	9.73	8.80 mm	8.8 mm	Small hydraulic adaptors, grease nipples, air line fittings
1/4" BSP	19	13.16	11.80 mm	11.8 mm	Hydraulic pressure gauges, small pipe connections, oil drain plugs
3/8" BSP	19	16.66	15.00 mm	15.0 mm	Hydraulic system fittings, fuel connections, coolant pipes
1/2" BSP	14	20.96	18.60 mm	18.5 mm	Most common size on UK farm machinery — hydraulics, air lines, water
5/8" BSP	14	22.91	20.60 mm	20.5 mm	Less common — some hydraulic manifolds and pump ports
3/4" BSP	14	26.44	24.30 mm	24.0 mm	Common on tractor hydraulics, slurry equipment, large pipe connections
7/8" BSP	14	30.20	27.80 mm	28.0 mm	Larger hydraulic valve blocks and manifolds
1" BSP	11	33.25	30.50 mm	30.5 mm	Main hydraulic supply lines, large bore pipe connections
1-1/4" BSP	11	41.91	39.20 mm	39.0 mm	Slurry tanker fittings, large bore irrigation and water supply
1-1/2" BSP	11	47.80	45.10 mm	45.0 mm	Large bore pipework — slurry, water supply, pneumatic mains
2" BSP	11	59.61	57.20 mm	57.0 mm	Large bore manifolds, bulk water connections, tank fittings