Dash sizes explained: what -08 actually means

A dash number is tube, in sixteenths

A dash number is the outside diameter of the TUBE the fitting serves, expressed in sixteenths of an inch. A -08 serves 8/16 in tube, which is half-inch tube. A -12 serves 12/16, which is three-quarter inch tube. That is the entire rule.

It says nothing about the thread. The thread on a -08 JIC is 3/4-16 UNF, which is 19.05 mm across the crests - half as big again as the tube it feeds. The thread on a -08 ORFS is 13/16-16, bigger still, on the same half-inch tube.

It also says nothing about flow. Two fittings of the same dash in different families have different bores, and a jump-size adapter is common enough that the dash number of one end tells you nothing about the other.

Why ORFS is always a size up

An ORFS male needs a flat face big enough to carry an O-ring groove outside the bore. That face has to sit somewhere, and the somewhere is a larger thread. So at every dash size the ORFS thread is one step above the JIC thread for the same tube.

The practical consequence is that ORFS and JIC of the same dash will not assemble at any size, which is the one good thing about it: the mistake catches itself at the counter rather than in the field.

Pipe trade sizes name a bore nobody has

NPT, BSPP and BSPT are sized by trade size, which descends from the nominal bore of wrought iron pipe in the nineteenth century. It has not matched a real dimension on the fitting for a long time.

A 1/4 in NPT has a basic major diameter of 0.540 in, or 13.72 mm. A G 1/4 BSPP is 13.157 mm. Neither is a quarter of an inch anywhere on the part. The rule of thumb that survives is to subtract about a quarter of an inch, roughly 6 mm, from a pipe-thread measurement to recover its trade size.

Because the two systems descend from different pipe traditions, an NPT and a BSPT of the same trade size are close but not the same, and at 1/2 and 3/4 in they even share a thread count. That is the trap the NPT against BSPT comparison covers in detail.

Metric fittings carry two numbers

A DIN 2353 fitting is described by the tube it serves in millimetres AND by its series letter, and the thread is a third number that follows from the pair. A 12L is 12 mm tube in the light series, and it runs an M18 x 1.5 thread. A 10S is 10 mm tube in the heavy series, and it runs the same M18 x 1.5 thread.

So the thread on a metric 24 degree fitting is not a size at all - seven of the threads in the standard serve one tube in the light series and a different one in the heavy series. The cutting ring is bored for one tube diameter and will not grip the other, so ordering by thread alone puts the wrong ring on the job.

Komatsu fittings are simpler in this respect and more awkward in another: they are named by their metric thread, and the two identification charts that carry them disagree about which tube some of those threads serve. Where they disagree, this site publishes no tube size at all.

Reading a part number

Most adapter part numbers encode the dash size of each end and a code for the end types. A number like 6400-08-06 is typically an -08 end and an -06 end on the same body: a jump-size adapter, which is why the working pressure of the assembly is the LOWER of the two ends rather than the higher.

That last point is stated by both pressure catalogues this site cites, and it is worth repeating because it is easy to lose: a fitting is only ever as strong as its weakest end, and on most adapters carrying a pipe thread, the pipe end is the weak one.