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

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.

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

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 vehicles and machines.

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

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.

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

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 fleet and plant machinery — found on 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.

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

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 plant and tractor hydraulic systems and larger pipe connections.

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

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.

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

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 tanker fittings and large bore water supply connections.

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

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 bulk liquid transfer, water supply mains, and pneumatic systems.

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

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.

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

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.

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

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 fleet and plant machinery.

What percentage thread engagement do these BSP drill sizes give?

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.

Where do I find BSP threads on fleet and plant machinery?

BSP threads are used throughout UK fleet and plant machinery wherever pipes, hoses, and fittings connect. Common locations include: 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), tank and bowser fittings (3/4" to 2" BSP), 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.

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

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.

What cutting fluid should I use when tapping BSP threads?

For steel, use a dedicated tapping fluid such as 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 is acceptable, though dedicated aluminium cutting fluid gives better results. Never tap dry in steel — it is the most common cause of tap breakage.

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Free Reference Calculator

BSP Tap Drill Size Calculator

Name: FleetBolt
Author: FleetBolt

Correct tapping drill size for every BSP pipe thread — 1/8" to 2" — for UK fleet, plant and workshop use.

Store fitting sizes and key specs against every asset in your fleet — free.

See How

Most searched BSP sizes — tap to select

BSP Tapping Drill Size Lookup

Select BSP pipe size

Tapping Drill

18.60mm

nearest std: 18.5mm

Thread OD

20.96mm

TPI

threads/inch

1/2" BSP is used for: Most common size on fleet & plant machinery — hydraulics, air lines, waterRemember: BSP nominal sizes do not correspond to measured dimensions — always use the table, never guess.

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

Click any row to select that size. All dimensions in millimetres. Tapping drill sizes for 75% thread engagement.

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 fleet & plant 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 plant and tractor hydraulics, tanker 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	Tanker fittings, large bore irrigation and water supply
1-1/2" BSP	11	47.80	45.10 mm	45.0 mm	Large bore pipework — bulk liquid transfer, water supply, pneumatic mains
2" BSP	11	59.61	57.20 mm	57.0 mm	Large bore manifolds, bulk water connections, tank fittings

Standard BSP (British Standard Pipe) parallel threads per BS 21. Tapping drill sizes for ~75% thread engagement in steel and cast iron.

Where Do BSP Threads Appear on Fleet & Plant Machinery?

BSP (British Standard Pipe) is the dominant pipe thread standard on UK-manufactured and European fleet and plant machinery. You will encounter BSP threads anywhere pipes, hoses, or fittings connect — on tractors, plant, commercial vehicles, and water/liquid supply systems.

BSP Size	Typical Location on Fleet & Plant 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	Hydraulics, air lines, water connections — most common
3/4" BSP	Hydraulic supply lines, agitators, bowsers
1" BSP	Main hydraulic return lines, large bore pipe joints
1-1/4" BSP	Tanker fittings, irrigation mains
1-1/2" BSP	Large bore water and bulk liquid pipework
2" BSP	Tank fittings, bulk water connections, manifold ports

Identifying an unknown BSP fitting: Measure the outside diameter of the male thread with a vernier caliper and match against the OD column in the table above. You can also count threads per inch using a thread pitch gauge — 28 TPI is always 1/8" BSP; 19 TPI is 1/4" or 3/8"; 14 TPI is 1/2" to 7/8"; 11 TPI is 1" and larger.

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

Both BSP thread forms use the same tapping drill size for each nominal size — the drill table above applies to both. The difference is in what the thread does, not how it's cut.

BSPP — Parallel (G Thread)

Thread diameter is constant along its length. The most common form on modern UK and European fleet and plant machinery, hydraulic fittings, and plumbing. Sealing is achieved with a bonded seal, O-ring, or PTFE tape on the threads. The 'G' prefix on fittings (e.g. G1/2) indicates parallel BSP.

BSPT — Tapered (R/Rc Thread)

The thread tapers at 1 in 16 (3.58°) so the thread form itself creates the seal when tightened. Less common on modern fittings but found on older cast iron bodies, some gas fittings, and older hydraulic systems. The 'R' prefix indicates tapered BSP male; 'Rc' for tapered female.

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BSPP and BSPT threads are not reliably interchangeable under pressure — a BSPT male into a BSPP female port will engage but may not seal correctly. Always match like for like or use an appropriate adapter with bonded seal.

How to Tap a BSP Thread — Step by Step

Look up the tapping drill size in the table above — never estimate from the nominal size name.

Centre-punch the location accurately. Drill a 3–4 mm pilot hole first on thick or hard material.

Drill to the exact tapping diameter at slow speed with cutting fluid. Use a pillar drill for accuracy wherever possible.

Start the thread with a taper tap (marked with a long chamfer). Keep the tap square to the face — use a try-square to check.

Half turn forward, quarter turn back to break the chip. Re-apply cutting fluid every 2–3 turns.

Follow with a plug (second) tap, then a bottoming tap if you need threads close to the base of a blind hole.

Test with an actual BSP fitting — it should thread smoothly by hand for 3–4 turns before resistance builds.

Tap sequence: Taper tap (1) → Plug tap (2) → Bottoming tap (3) for blind holes. For through-holes in thin material, a single plug tap is usually sufficient. Never tap dry in steel — cutting fluid is essential.

Frequently Asked Questions — BSP Tap Drill Sizes

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