Hydraulic Fittings and Thread Standards (BSP/Metric)
A hydraulic fitting must match the thread, the sealing method and the pressure. Confusing BSP with NPT or metric leads to leaks, damaged threads and downtime. This guide shows how to identify thread standards reliably, work out the sealing method and pick the right size.
View fittingsWhich thread standards exist in hydraulics?
Several thread systems meet in hydraulics. BSP (Whitworth, G and R) has a 55° flank angle and is common in Europe. Metric threads (M) with a 60° flank angle are the standard for tube fittings to ISO 8434 (DIN 2353). NPT and NPTF are tapered US inch threads at 60°. The systems are not compatible, even though they may start by hand.
How does a hydraulic fitting seal correctly?
The sealing method decides tightness and reusability. On parallel threads (G, M) the thread does not seal itself; a sealing element does: an O-ring on the port face (ED seal) or a soft sealing cone. Tapered threads (R, NPT) seal metal to metal in the thread and need extra sealant. For high pressure, O-ring sealed systems are usually the more robust choice.
- Parallel G/M with O-ring (ED): tight at high pressure, often removable and reusable.
- Tapered R/NPT: seals on the thread flanks, needs PTFE tape or anaerobic sealant.
- Cutting-ring fitting to ISO 8434: metal on metal at the tube, for rigid pipework.
- 24° flare and flanges (SAE) for very high pressure and large nominal sizes.
How do I work out size and tightening torque?
Inch labels on BSP refer to a historic pipe bore, not the measured thread: G 1/2 measures about 21 mm outer diameter. Measure the thread outer diameter with a caliper and count the threads with a thread gauge. Tightening torque depends on size, material and sealing method and must be taken from the maker's data.
Frequently asked questions
Does a BSP thread fit an NPT thread?
No. BSP has a 55° flank angle, NPT 60°, and the pitch differs too. They often start by hand but will not seal reliably and damage the flanks. Use a proper BSP-to-NPT adapter for transitions.
Why does my fitting leak even when tight?
Usually the sealing method is wrong: a parallel G thread seals via an O-ring, not the flanks. Over-tightening will not make up for a missing seal element; it only damages thread and sealing face. Check the O-ring, sealing edge and the specified torque.
What does G 1/2 mean in millimeters?
The inch figure is a historic nominal size, not a measurement. A G 1/2 thread has about 21 mm outer diameter and 14 threads per inch. To identify it, measure the outer diameter and confirm the pitch with a thread gauge.
Find the right hydraulic fitting
Our hydraulics specialists help with thread standard, sealing method and size for your system.
Standard-compliant choice
Recommendations to ISO 8434 and maker data.
Correct size
Measure and identify the thread with confidence.
Tight and safe
The right sealing method for lasting leak-free joints.
Expert advice
Hydraulics specialists support standard and adapter choice.