British Standard Pipe fittings are among the most popular of all foreign threads in the world today. These threads come in two versions: parallel (BSPP), and tapered (BSPT). The thread flank angle for both tapered and parallel British threads is 55°. Although BSP is a foreign thread, it isn’t actually metric. 
This is why it comes in Imperial sizes: 1/8, 1/4, 1/2, 3/4, etc.

BSP parallel (BSPP) threads commonly seal via a 30° chamfer on the male thread to a 30° cone seat inside the female thread (swivel only). If it is a port application, an O-ring and washer, Elastomeric seal or a bonded washer are needed to achieve a proper seal for parallel threads. For tapered BSP threads (BSPT), a seal is accomplished via thread wedging with additional support from thread dope or tape. As previously mentioned, it is possible for a male BSPT (tapered) to thread into a female BSPP (parallel), so long as the female thread is fixed and not swivel (this is because the recessed cone seat could interfere with clearance). 

To identify a BSPP fitting complete the following steps:

Use the diagram below to confirm what type of British fitting you have. Use caution as “multi-purpose fittings” exist that accommodate 2 or more sealing styles in one fitting (such as Adaptall’s 9005ES series that can be used for BSPP port or swivel connection).

British Standard Pipe Parallel

A captive seal is made using metal to metal angled surfaces, or a combination of metal to metal with an o-ring. Female swivel BSPP has a tapered nose flareless swivel where the seal occurs on the cone seat of the male connector. Similar but not interchangeable with NPSM/NPT since most sizes will have a different thread pitch and OD and thread forms are close, but not the same. Thread sealant is recommended when securing this connection style. 

British Standard Pipe Tapered

A captive seal is made using metal to metal angled surfaces, or a combination of metal to metal with an o-ring. Female swivel BSPP has a tapered nose flareless swivel where the seal occurs on the cone seat of the male connector. Similar but not interchangeable with NPSM/NPT since most sizes will have a different thread pitch and OD and thread forms are close, but not the same. Thread sealant is recommended when securing this connection style. BSPT male adapters are compatible with both BSPP and BSPT female ports.

NOTE: For British tapered threads (BSPT) caution should be taken as the most common tapered fitting in North America is NPT. The easiest way to distinguish any tapered fitting type from NPT is to verify if a chamfered sealing surface exists. If it does the fitting is most likely NPT. If not, it is either BSPT or Metric Taper (Metric Taper is far less common).

SAE J518 4-Bolt Code 61 Flange

The Code 61 Flange is the standard 3000psi rated SAE J518 4-Bolt Flange. For higher pressure, use the 6000psi rated Code 62 flange. The overall design for the Code 61 & Code 62 Flanges is the same, but the flange head diameters and bolt hole spacing are larger for Code 62 flanges. The female port of the fitting is a smooth, unthreaded port with four bolt holes set in a rectangular pattern around the port. The male is a flanged head with a groove for an O-ring and either split or captive flange halves and bolt holes which match the port. The seal is made where the O-Ring is compressed between the flanged head and the flat surface of the port. The connection is held by threaded bolts.

SAE J518 4-Bolt Code 62 Flange

The Code 62 Flange is the standard 6000psi rated SAE J518 4-Bolt Flange. For lower pressure, use the 3000psi rated Code 61 flange. The overall design for the Code 61 & Code 62 Flanges is the same, but the flange head diameters and bolt hole spacing are larger for Code 62 flanges. The female port of the fitting is a smooth, unthreaded port with four bolt holes set in a rectangular pattern around the port. The male is a flanged head with a groove for an O-ring and either split or captive flange halves and bolt holes which match the port. The seal is made where the O-Ring is compressed between the flanged head and the flat surface of the port. The connection is held by threaded bolts.

National Pipe Tapered

NPT is one of our most popular thread styles. NPT is a USA standard for tapered threads used on pipes and fittings. They are used to effectively seal pipes for fluid and gas transfer. Nominal pipe size can be identified by physically measuring the thread diameter, then subtracting 1/4". NPT connections rely on thread deformation, a metal to metal sealing design where the threads of the connectors themselves form together. This design is ideal for single assembly applications, and not recommended where connections will be assembled and disassembled frequently. Do not over-tighten. We recommend using a thread sealant with this type of connection style. 

National Pipe Tapered Fuel

Where traditional NPT threads have a small clearance between the crest and root of the mating threads requiring the use of sealant, the NPTF thread root does not allow this clearance. It contacts and deforms the mating thread crest. NPTF, or National Pipe Taper Fuel, style connections are widely used in fluid power systems. This type of connector is designed to provide a dry thread seal. However, lubricant is often still used to ease assembly. Most of the NPT adapters sold on OneHydraulics.com are NPTF style, but called out as just 'NPT'. 

National Pipe Straight Mechanical

NPSM, or National Pipe Straight Mechanical threads, are also often found in fluid power systems. The female component incorporates a straight, non-tapered thread with an inverted 30° chamfer of the male connector on the seat of the female. This is a mechanical seal. If and NPTF male is properly chamfered it will also seat with NPSM female connection. They are especially suited for manufacturers and end users that rely on versatility.

Joint Industry Council SAE J514

Both male and female JIC components are easily identified by their 37° tapered seats. This connection style is very popular in fluid power, especially hose end connections. The seal is made by establishing contact between the male flared and female coned seat. This is considered a mechanical connection. There are also SAE J512 45° connections, but those are more typical in automotive, refrigeration, and truck pipe systems. The JIC sold on OneHydraulics.com is the 37° style.

Straight Thread O-Ring Boss SAE J1926

Often called an "SAE" or "ORB" port, this thread style is recommended by the National Fire Protection Association (NFPA) for leak prevention. The male connection is a straight thread with an O-ring. The female port has a straight thread and a machined surface to provide a smooth, flat surface (minimum spot-face), along with a chamfer where the O-ring seats. It seals when the O-ring is compressed into the chamfer when mating the male connection. This is also considered a mechanical connection. 

O-Ring Face Seal SAE J1453

O-ring Face Seal connections are considered the best for leak control. The male connector has a straight thread and an O-ring in the face. The female has a straight thread and a machined flat face. The seal takes place by compressing the O-ring onto the flat face of the female, similar to the split flange type fitting. The threads maintain the connection mechanically. 

DIN fittings have a 24° tapered throat angle that exists inside the male threads. To achieve a tube connection, a tube nut is tightened over the metric threads (hardened pre-assembly fitting bodies are recommended for installation) forcing a ferrule or cutting ring to grip the tube. By having the nut securely fastened to the tube fitting, both compress the cutting ring which holds the assembly in place and creates a seal. This sealing style also includes Kobelco fittings which are essentially an L-series Metric thread with a 1.5mm thread pitch in all sizes.

Metric threads share many characteristics with British threads, so caution must be taken when attempting to differentiate the two. First, if the Metric thread is DIN 7631 it will have a 30° chamfer on the male thread, the same as a BSPP male. For fittings in port application, Metric fittings (ISO 9974) also mimic BSPP (ISO 1179) stud ends with the only difference being the threads. BSPT fittings closely resemble Metric taper fittings, although Metric taper is far less common. There are also variations of the sealing surfaces among different Metric fitting types that can create difficulty when attempting to identify.

If you have a Metric tapered thread, one would undergo the same initial steps for identification of thread pitch as a parallel Metric thread. Also similar to parallel Metric threads is that an O.D measurement will reveal the actual thread size (i.e. 18.01mm measurement means the thread is likely M18). However, to do this accurately with Tapered Metric threads one must measure the O.D at the third row of threads from the end of the fitting.