Hydraulic adapter end threads by dash size: JIC 37° flare by tube OD, SAE O-ring boss and the pipe end.
A hydraulic adapter is sized by its dash number — sixteenths of an inch. On a JIC 37° flare or O-ring boss end the dash is the tube OD: a -6 end is for 3/8″ tube and is 9/16-18. On a pipe end the dash is the pipe size: -4 is 1/4″ pipe, 1/4-18. The threads per inch and size of a pipe end are on the pipe thread chart; brass flare and compression threads are on the tube fitting chart.
| Tube OD | Dash size | Thread |
|---|---|---|
| 1/8″ | -2 | 5/16-24 |
| 3/16″ | -3 | 3/8-24 |
| 1/4″ | -4 | 7/16-20 |
| 5/16″ | -5 | 1/2-20 |
| 3/8″ | -6 | 9/16-18 |
| 1/2″ | -8 | 3/4-16 |
| 5/8″ | -10 | 7/8-14 |
| 3/4″ | -12 | 1-1/16-12 |
| 7/8″ | -14 | 1-3/16-12 |
| 1″ | -16 | 1-5/16-12 |
| 1-1/4″ | -20 | 1-5/8-12 |
| 1-1/2″ | -24 | 1-7/8-12 |
| 2″ | -32 | 2-1/2-12 |
| Dash size | Tube OD | Thread |
|---|---|---|
| -2 | 1/8″ | 5/16-24 |
| -3 | 3/16″ | 3/8-24 |
| -4 | 1/4″ | 7/16-20 |
| -5 | 5/16″ | 1/2-20 |
| -6 | 3/8″ | 9/16-18 |
| -8 | 1/2″ | 3/4-16 |
| -10 | 5/8″ | 7/8-14 |
| -12 | 3/4″ | 1-1/16-12 |
| -14 | 7/8″ | 1-3/16-12 |
| -16 | 1″ | 1-5/16-12 |
| -20 | 1-1/4″ | 1-5/8-12 |
| -24 | 1-1/2″ | 1-7/8-12 |
| -32 | 2″ | 2-1/2-12 |
| Dash size | Pipe size | Thread |
|---|---|---|
| -2 | 1/8″ | 1/8-27 |
| -4 | 1/4″ | 1/4-18 |
| -6 | 3/8″ | 3/8-18 |
| -8 | 1/2″ | 1/2-14 |
| -12 | 3/4″ | 3/4-14 |
| -16 | 1″ | 1-11½ |
| -20 | 1-1/4″ | 1-1/4-11½ |
| -24 | 1-1/2″ | 1-1/2-11½ |
| -32 | 2″ | 2-11½ |
Good to know
Size by the dash number. The dash number is the size in sixteenths of an inch: on a JIC or O-ring boss end it is the tube OD (-6 = 3/8″ tube), on a pipe end the pipe size (-4 = 1/4″ pipe). Two ends of one adapter can be different dash sizes.
37° flare (JIC). The male end has a 37° cone; the female swivel nut pulls the flared tube or the female seat onto it and the metal-to-metal cone makes the seal — the thread only holds it together. A 45° SAE flare is a different seat even where the thread is the same.
O-ring boss. A straight thread with an O-ring at the base of the male end; the O-ring seals in the chamfer of the port. Never put a pipe or JIC fitting in an O-ring boss port.
Pipe end. A taper thread that seals on the threads; hydraulic adapter makers list it as NPTF (Dryseal), the same threads per inch and size as NPT. Use thread sealant on the male thread.
Same thread, different end. A JIC end and an O-ring boss end of the same dash size share a thread (-4 is 7/16-20 on both) but seal differently — match the end type, not just the thread.
Ratings come from the maker. Working pressure depends on the end type, the size, the material and the maker — use the maker’s rating for that exact fitting. This chart gives sizes only.
Quick answers
- What thread is a -6 JIC fitting?
- A -6 JIC 37° flare end (3/8″ tube OD) is 9/16-18; -4 is 7/16-20 and -8 is 3/4-16.
- What thread is a -8 O-ring boss (ORB) port?
- A -8 SAE O-ring boss is 3/4-16; -4 is 7/16-20 and -6 is 9/16-18.
- Is -6 JIC the same thread as -6 O-ring boss?
- Yes — both are 9/16-18 — but they seal differently: the JIC end on a 37° cone, the O-ring boss end with an O-ring in the port. They do not seal in each other’s place.
- What does -12 mean on a hydraulic fitting?
- The dash number is the size in sixteenths of an inch: on a JIC or O-ring boss end it is the tube OD, so -12 = 3/4″ tube (1-1/16-12); on a pipe end it is the pipe size.
- What is the pipe thread on a -4 hydraulic adapter?
- A -4 pipe end is 1/4-18 (1/4″ pipe) and -6 is 3/8-18 — hydraulic adapter makers list it as NPTF (Dryseal), the same threads per inch as NPT.