Why Getting Thread Identification Right Matters
Hydraulic fitting identification is one of the most common sources of error in hydraulic systems maintenance. Using the wrong fitting or adaptor can result in cross-threading, leaks, and in pressurised systems, potentially dangerous failures. With several competing thread standards in common use across UK industry, knowing how to measure and identify threads correctly is an essential skill.
The Main Thread Standards Used in UK Hydraulics
BSP (British Standard Pipe)
BSP is the dominant thread standard in UK and European hydraulic systems and comes in two forms:
- BSPP (Parallel): Also known as BSP-G or G thread. The thread form is parallel — it does not taper. Sealing is achieved via a bonded seal (Dowty washer), a soft metal washer, or an O-ring face seal (ORFS), not through the thread itself. Used on most UK hydraulic ports and fittings.
- BSPT (Tapered): A tapered thread that creates a pressure seal through thread interference as it is tightened. Less common in hydraulics but found on some older equipment. Can be confused with BSPP visually — always check.
NPT / NPTF (National Pipe Taper)
NPT is the North American standard and is tapered like BSPT but uses a different thread form (60° vs 55° for BSP). It is found on imported equipment from the USA and some Asian manufacturers. NPT and BSP threads are not interchangeable — do not attempt to mix them.
JIC (Joint Industry Council) / AN Fittings
JIC fittings use a 37° flare seat for sealing and are common in aerospace, motorsport, and some mobile hydraulic applications. They look similar to AN fittings and use the same thread form but may differ in material specification. The 37° flare provides a reliable metal-to-metal seal without the need for additional sealing elements.
Metric Threads (DIN/ISO)
Metric fittings are increasingly common, particularly on German and Japanese equipment. They use an O-ring face seal (ORFS) or a 24° cone seat for sealing. Always check the thread pitch alongside the nominal diameter to avoid confusion with UNF threads of similar size.
How to Identify BSP Thread Size
BSP thread sizes are a legacy sizing system where the nominal size does not correspond directly to the actual thread diameter. Here is how to measure correctly:
- Measure the thread outside diameter (OD) with calipers: For male threads, measure across the thread crests. For female threads, measure across the thread roots (inner diameter).
- Count the threads per inch (TPI): Use a thread pitch gauge for accuracy. Count the number of thread crests across one inch of the thread.
- Cross-reference with a BSP size chart: The nominal BSP size does not equal the measured OD. For example, a 1/2″ BSP male thread measures approximately 20.96mm OD with 14 TPI.
Common BSP sizes and approximate male thread ODs:
- 1/8″ BSP: ~9.73mm OD, 28 TPI
- 1/4″ BSP: ~13.16mm OD, 19 TPI
- 3/8″ BSP: ~16.66mm OD, 19 TPI
- 1/2″ BSP: ~20.96mm OD, 14 TPI
- 3/4″ BSP: ~26.44mm OD, 14 TPI
- 1″ BSP: ~33.25mm OD, 11 TPI
Male vs Female Fittings
This seems simple but is worth stating clearly for anyone new to hydraulic fittings:
- Male (external thread, BSPM or BSPP-M): The thread is on the outside of the fitting. Male fittings screw into female ports.
- Female (internal thread, BSPF or BSPP-F): The thread is inside the fitting body. Female fittings receive male threads.
When ordering adaptors, you need to specify the size and gender of both ends. For example, a “1/2″ BSPP male to 3/8″ BSPP female adaptor” is a reducing adaptor that accepts a 3/8″ male in one end and screws into a 1/2″ female port.
Parallel vs Tapered: How to Tell Them Apart
Visual inspection alone is unreliable for distinguishing BSPP from BSPT. The most reliable method is to use a straight edge along the thread profile: BSPP threads run parallel to the axis; BSPT threads taper visibly towards the end. A thread gauge or thread gauge set will confirm the form and pitch definitively.
Browse Hydraulic Adaptors and Fittings
Hydra-Line stocks an extensive range of hydraulic adaptors and fittings, including BSP, metric, JIC, and NPT types in a wide range of sizes and configurations. If you need a specific adaptor or are unsure what you require, our team can help you identify the correct fitting quickly.
Call 01782 658535 or email info@hydra-line.co.uk with your measurements and we will identify and source the right fitting for you.