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 the thread was designed for in the 19th century — not the outside diameter of the thread. A 1/2" BSP fitting has a thread OD of 20.955 mm (0.825"). Always measure the OD with calipers 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 calipers to measure the outside diameter (OD) of the pipe or fitting thread, then find the closest match in the table for the pipe type you are working with. For BSP fittings, measure across the thread OD — never the ID.

Question 3

What is the difference between BSPP and BSPT?

Accepted Answer

BSPP (parallel) relies on a face seal, copper washer or O-ring for a pressure-tight joint. BSPT (taper) tightens as it is screwed in and seals on the thread itself, usually with PTFE tape. Both share the same pitch and nominal sizing — only the thread form differs.

Question 4

What pipe is used for site water supply?

Accepted Answer

Blue MDPE (Medium Density Polyethylene) is the UK standard for buried potable water supply, available 20mm to 160mm OD to SDR11 spec, rated to 12.5 bar at 20°C. Black HDPE pipe covers non-potable applications.

Question 5

What is SDR in MDPE pipe?

Accepted Answer

SDR (Standard Dimension Ratio) is the ratio of pipe OD to wall thickness. SDR11 means OD ÷ wall thickness = 11. A lower SDR means a thicker wall and higher pressure rating.

Question 6

What hydraulic tube should I use for plant machinery repair?

Accepted Answer

Use seamless cold-drawn steel tube to DIN 2391 or EN 10305-4. Match wall thickness and OD to the tube being replaced — increase wall thickness for higher working pressures. Never use copper for hydraulic circuits; it work-hardens and cracks under vibration.

Nominal Size	OD (mm)	OD (in)	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 plant/fleet 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, 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, general pipework
2"	59.614	2.347"	11	50.0 mm	Large bore water systems
2-1/2"	75.184	2.960"	11	63.5 mm	Major water infrastructure
3"	87.884	3.460"	11	76.0 mm	Large diameter water supply
4"	113.03	4.450"	11	100.5 mm	Major infrastructure, underground

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