JIC 37° vs SAE 45° Flare Fittings
JIC and SAE 45° flare fittings share threads in several sizes but use different seat angles. This guide shows which sizes overlap and how to tell them apart.
Last updated October 30, 2026. Reference values; verify against manufacturer specifications.
JIC 37° and SAE 45° flare fittings look almost the same. Both use a cone-shaped flare, both use straight inch threads, and in six dash sizes the threads are identical. The difference is the seat angle, and that 8° difference decides whether a joint seals or leaks.
The short answer
- JIC 37° flare is defined by SAE J514. It is the standard flare for hydraulic hose and tube.
- SAE 45° flare is defined by SAE J512. It is common on fuel, refrigeration, gas, and other lower-pressure lines, often in brass or soft copper tubing.
- The seat angles are different, so a 37° part and a 45° part do not seal against each other properly, even when the threads match.
What the angles mean
Both angles are measured from the centerline of the fitting. A JIC 37° cone therefore has an included angle of 74°, and a 45° cone has an included angle of 90°. When a 37° seat is pressed against a 45° seat, the two cones only touch along a thin ring instead of across a full band. That narrow contact can hold briefly at low pressure, then weep or blow out as pressure, heat or vibration increase. It can also deform the seats so that neither part seals well again afterward.
Thread comparison by dash size
This is where the confusion starts. In sizes -2, -3, -4, -5, -8 and -10 the threads are the same, so mismatched parts screw together and feel normal. In sizes -6, -12, -14 and -16 the threads differ.
| Dash | JIC 37° thread (SAE J514) | SAE 45° thread (SAE J512) | Same thread? |
|---|---|---|---|
| -2 | 5/16-24 | 5/16-24 | Yes |
| -3 | 3/8-24 | 3/8-24 | Yes |
| -4 | 7/16-20 | 7/16-20 | Yes |
| -5 | 1/2-20 | 1/2-20 | Yes |
| -6 | 9/16-18 | 5/8-18 | No |
| -8 | 3/4-16 | 3/4-16 | Yes |
| -10 | 7/8-14 | 7/8-14 | Yes |
| -12 | 1-1/16-12 | 1-1/16-14 | No (same diameter, different pitch) |
| -14 | 1-3/16-12 | 1-1/4-12 | No |
| -16 | 1-5/16-12 | 1-3/8-12 | No |
A few things to notice:
- -6 is the classic trap in reverse. SAE 45° -6 uses 5/8-18, while JIC -6 uses 9/16-18. The pitch is the same, so the diameter is what tells them apart.
- -12 is the subtle one. Both are 1-1/16 in diameter, but SAE 45° uses 14 TPI and JIC uses 12 TPI. Calipers alone will not separate them; a thread pitch gauge will.
- Common SAE 45° charts stop at -16. JIC continues to -20, -24 and -32 (see the full JIC size chart).
JIC vs SAE 45° at a glance
| Feature | JIC 37° flare | SAE 45° flare |
|---|---|---|
| Standard | SAE J514 (ISO 8434-2) | SAE J512 |
| Seat angle (from centerline) | 37° | 45° |
| Included cone angle | 74° | 90° |
| Thread type | Straight UNF (-2 to -10), UN 12-pitch (-12 and up) | Straight UNF/UN |
| Typical uses | Hydraulic hose and tube | Fuel, refrigeration, gas, automotive and lower-pressure lines |
| Typical materials | Steel, stainless steel, some brass | Often brass, with copper or soft tubing |
| Size range in common charts | -2 to -32 | -2 to -16 |
How to tell a 37° fitting from a 45° fitting
Judging the angle by eye is unreliable. The 45° cone looks a little blunter, but the difference is small, and wear or paint can hide it. Use this sequence instead:
- Measure the thread. Use calipers on the thread crests and a thread pitch gauge. If the result is 9/16-18 it could be JIC -6 (it cannot be SAE 45° -6, which is 5/8-18). If it is 1-1/16-14 it is SAE 45° -12, not JIC. For the overlap sizes, go to the next step.
- Use a seat angle gauge. Flare gauges usually include 37° and 45° profiles. Hold the matching side against the cone (male) or seat (female). The correct angle sits flush along the whole face, with the gauge's centerline parallel to the fitting's axis. The wrong one rocks or shows a light gap.
- Look at the context. A brass fitting on a fuel, gas or refrigeration line is more likely to be 45°. A plated steel fitting on hydraulic hose is more likely to be JIC 37°. Context is a hint, not proof; always confirm with the gauge.
- Check markings and part numbers. Many manufacturers identify flare angles in their part numbering or catalog descriptions. Look up the part where possible.
Step-by-step measuring tips are in how to measure JIC fittings.
What about dual-seat fittings?
Some female swivel fittings and some adapters are made with a seat designed to accept both 37° and 45° male cones. These are specific products, made and labeled for that purpose. A standard JIC or SAE 45° part should not be assumed to be dual-seat. If you are relying on a dual-seat fitting, confirm it from the manufacturer's catalog for that exact part, and check whether its pressure rating suits the application.
Where AN fittings fit in
AN (Army-Navy) fittings also use a 37° flare and the same thread sizes as JIC. They are made to military and aerospace specifications with a tighter thread class, and are widely used in motorsport. An AN fitting and a JIC fitting of the same dash size generally connect (see JIC vs AN fittings), while an AN fitting and an SAE 45° fitting have the same angle mismatch described above.
Why the mismatch matters
- Leaks under pressure. Line contact between mismatched cones is not a reliable seal for hydraulic pressures.
- Seat damage. Tightening harder to stop a leak flattens or cracks the cones, and then neither part seals properly with its correct mate.
- Hidden failures. Because the overlap sizes thread together smoothly, a mismatched joint may pass a visual check and only leak once the system runs hot.
If a JIC joint keeps leaking after correct assembly, a 37°/45° mismatch is one of the first things to check. Other common causes are covered in common JIC fitting mistakes.
References
Thread data is cross-checked against published manufacturer and distributor charts, including the Eaton Thread Identification Guide (which cautions that 37° and 45° fittings share threads in some sizes), the Brennan thread and port identification guide, Pulsar's thread identification tables and QC Hydraulics' SAE 45° chart. Always verify part details against the fitting manufacturer's specifications.
Frequently asked questions
Can I connect a JIC 37° fitting to an SAE 45° fitting?
Not reliably. In some sizes the threads match and the parts screw together, but the 37° and 45° seats only touch along a thin line and may leak or fail under pressure. Use matching seats or a proper adapter.
Which JIC and SAE 45° sizes have the same threads?
Dash sizes -2, -3, -4, -5, -8 and -10 have the same threads. Sizes -6, -12, -14 and -16 differ.
What thread is a -6 SAE 45° flare?
A -6 SAE 45° flare uses a 5/8-18 thread. A -6 JIC 37° fitting uses 9/16-18.
How do I check a flare angle?
Use a seat angle (flare) gauge with 37° and 45° profiles. The correct profile sits flush on the cone or seat with the gauge parallel to the fitting's centerline.
Is a 45° flare stronger than a 37° flare?
They are designed for different jobs. JIC 37° is the standard for hydraulic hose and tube; SAE 45° is mainly used for lower-pressure lines. Pressure ratings depend on size, material and manufacturer.