Question 1

Why does a 1/2" BSP fitting not measure 1/2 inch?

Accepted Answer

BSP (British Standard Pipe) sizing is based on the internal bore of the original wrought iron gas pipe that the thread was designed for in the 19th century — not the outside diameter of the thread. A 1/2" BSP fitting refers to a pipe with an approximate 1/2" bore, but the thread OD is actually 20.955mm (0.825"). This confuses everyone the first time — always measure the OD with callipers and look up the nominal size from the table.

Question 2

How do I measure a pipe to identify its size?

Accepted Answer

Use digital callipers to measure the outside diameter (OD) of the pipe or fitting thread. Then find the closest match in the reference table for the pipe type you are working with. For BSP fittings, measure across the thread OD. For copper and hydraulic tube, measure the actual tube OD. For MDPE water pipe, measure the OD of the pipe wall. Never measure the ID to identify a BSP fitting — always the OD.

Question 3

What is the difference between BSPP and BSPT?

Accepted Answer

BSPP (British Standard Pipe Parallel) has a parallel thread — the thread diameter is the same along its full length. It relies on a bonded seal, copper washer or O-ring face seal to achieve a pressure-tight joint. BSPT (British Standard Pipe Taper) has a tapered thread that tightens as it is screwed in, sealing on the thread itself (usually with PTFE tape). Both have the same thread pitch and nominal sizing — only the thread form differs. BSPP is most common on hydraulic fittings; BSPT on water and gas.

Question 4

What pipe is used for farm water supply in the UK?

Accepted Answer

Blue MDPE (Medium Density Polyethylene) is the standard for buried potable water supply in the UK. It is available in 20mm to 160mm OD to SDR11 specification, rated to 12.5 bar at 20°C. The blue colour designates potable (drinking) water use. Black HDPE pipe is used for non-potable applications. For above-ground farm water supply, copper or MDPE are both used depending on the application and exposure to frost.

Question 5

What is SDR in MDPE pipe?

Accepted Answer

SDR stands for Standard Dimension Ratio — it is the ratio of the pipe OD to the wall thickness. SDR11 means the OD divided by the wall thickness equals 11. A lower SDR number means a thicker wall and higher pressure rating. SDR11 MDPE blue water pipe is rated to 12.5 bar, which is sufficient for most farm water supply applications including mains pressure and moderate pumped systems.

Question 6

What hydraulic tube should I use for farm machinery repair?

Accepted Answer

For most farm hydraulic repair work, use seamless cold-drawn steel tube to DIN 2391 or EN 10305-4 specification. The wall thickness and OD should match the original tube being replaced. For working pressures up to 250 bar, 1.5mm wall is adequate for 10-15mm OD tube. For higher pressure applications (power steering, loader circuits), increase wall thickness to 2mm or 2.5mm. Always use appropriate flare fittings or compression fittings rated for the working pressure.

Question 7

Can I use copper pipe for hydraulic systems on farm machinery?

Accepted Answer

No — copper pipe is not suitable for hydraulic systems on farm machinery. Copper work-hardens under the vibration and pressure cycles of a hydraulic system and will eventually crack and fail. Always use seamless steel hydraulic tube for hydraulic circuits. Copper is suitable for fuel lines (where it has been used historically), low-pressure water and heating circuits only.

Question 8

What PTFE tape should I use on BSP fittings?

Accepted Answer

Use standard white PTFE thread seal tape for BSPT (tapered) fittings on water and hydraulic applications. For oxygen systems use yellow PTFE. For gas systems use yellow gas-rated PTFE. Never use PTFE tape on BSPP (parallel) fittings — these seal on a face or O-ring, not the thread. Wrapping PTFE on a parallel thread does not create a seal and can cause fittings to crack from overtightening.

Nominal Size	OD (mm)	OD (inches)	TPI	Approx Bore	Common Use
1/8"	9.728	0.383"	28	6.8 mm	Grease nipples, small instrument fittings
1/4"	13.157	0.518"	19	9.0 mm	Hydraulic gauges, small valves, brake pipes
3/8"	16.662	0.656"	19	12.5 mm	Common hydraulic fittings, fuel lines
1/2"	20.955	0.825"	14	16.0 mm	Most common farm hydraulic size, water
3/4"	26.441	1.041"	14	20.5 mm	Larger hydraulic lines, water supply
1"	33.249	1.309"	11	25.5 mm	Main supply lines, slurry valves
1-1/4"	41.91	1.650"	11	32.0 mm	Large bore hydraulic, water mains
1-1/2"	47.803	1.882"	11	38.5 mm	Water supply, slurry pipework
2"	59.614	2.347"	11	50.0 mm	Large bore water, slurry systems
2-1/2"	75.184	2.960"	11	63.5 mm	Major water and slurry infrastructure
3"	87.884	3.460"	11	76.0 mm	Large diameter slurry and water
4"	113.03	4.450"	11	100.5 mm	Major infrastructure, underground

Pipe Size Reference Chart

How to Identify a Pipe or Fitting

Frequently Asked Questions