There are four basic types of tube fittings: Flare, Flareless, Straight Thread O-Ring, and Flat faced O-Ring. Tube fittings seal in two ways. Flare and Flareless fittings use metal to metal contact joints. Straight Thread O-Rings and Flat Face O-Ring fittings use a rubber o-ring. Where extreme vibration is present, use Flareless, Straight Thread or Flat Face O-Ring Seal fittings.
Sizing: For accuracy, it is recommended the male thread be measured. Measure the outside diameter.
The American Society of Automotive Engineers in cooperation with industry set a standard for improvement in pipe threads. This improvement is known as the Dryseal Pipe Thread. All Weatherhead pipe threads are American Standard Taper Dryseal Pipe Threads (NPTF). The metal to metal seal is formed by contact at the thread crest and root.
Nominal pipe sizes do not agree with either the I.D., O.D., or thread sizes. To determine pipe size (up to 1 1/4") measure the diameter of the threads and subtract 1/4" from a 1" pope to obtain the nominal pipe size of 3/4". Pipe sizes can also be given in 'dash numbers.' A dash number is always the numerator of an inch over 16th. For instance, if the pipe O.D. measures 1/2" that would be converted to 16ths (8/16), but be written as -8.
BS = British Standards Institution
ISO = International Standards Organization
DIN = Deutsche Industrie Norme
To identify metric connections you will need instruments that can accurately measure thread inside and outside diameters, thread pitch, and fitting seat.
Parallel and Tapered Threads:
The first step in identifying thread forms is to determine if the thread is parallel or tapered. Parallel threads are not used for sealing fluids. Sealing is achieved by an elastomeric o-ring, metal seal, machined ring in the hex itself, or a seal machined into the end of the fitting. This style is similar to straight thread o-ring port connections where the threads are used for retention of the sealing method against a machine port. Parallel threads can be determined by laying a straight edge along the threads. If the threads are parallel to the center line of the fitting, then the fitting has parallel threads
Tapered threads seat by the interference caused by the male and female threads. These threads create a pressure-tight joint by metal deformation when they are tightened. Sealants on the threads are commonly used in this style of connection. Laying a straight edge on the threads, compare this line with the center line of the fitting. If this line tapers slightly away from the center line, then the threads are tapered.
British Pipe Threads: There are two forms of British Standard Pipe Threads that are used in the world today. They are BSPP (British Standard Pipe Parallel) and BSPT (British Standard Pipe Tapered). The BSPT male thread mates with the female BSPT thread similar to an NPTF connections. the 30 degrees BSPP male adapters connect to a female BSPP thread with a 30 degree cone. This style is comparable to an NPSM swivel style. These threads are almost identical to the NPTF Pipe Thread except for the flank angle. This angle is 55 degrees versus 60 degrees on the NPTF. Because of this difference the two forms are NOT interchangeable.
Metric threads are similar to inch-sized threads except for the sizing which is based on standard metric units. Identifying metric threads starts with determining if it is a parallel or tapered thread. Next measure the thread diameter. Then measure the pitch.