Flare Fittings: SAE 45° vs Inverted Flare | Apex Flow

Flare fittings seal metal-to-metal by clamping a cone-shaped tube end between a male and female seat — no thread sealant, no o-ring, and a connection that handles vibration and temperature cycling far better than a compression fitting. But "flare" covers several incompatible standards, and mixing them cross-seats and leaks. The two you will meet most in plumbing, refrigeration, gas, and automotive work are the SAE 45° flare and the inverted flare. Both use a 45° cone, yet they are not interchangeable. This guide explains how each seals, where each belongs, and how they differ from the 37° JIC flare used in hydraulics.

Apex Flow Solutions stocks brass SAE 45° and inverted flare fittings across the common tube sizes. Pressure figures below are representative for brass flare fittings; confirm on the specific data sheet.

Flare connection leaking or won't seat?

A leak almost always means a mismatched flare standard or angle. Tell our team your tube size, application, and the fitting you're mating to and we'll confirm the right flare type.

How a Flare Fitting Seals

To make a flare joint, the tube end is opened into a cone (flared) with a flaring tool. A flare nut slides over the tube, and tightening the nut presses the flared cone against the matching coned seat of the fitting body, creating a metal-to-metal seal. There is no gasket or sealant in the flow path, so the joint resists heat, vibration, and pressure cycling and can be disassembled and remade many times. The seal depends entirely on matching the cone angle and the seat geometry — a 45° flare on a 37° seat (or vice versa) contacts on only a thin line and leaks. The flare angle is therefore the single most important compatibility spec.

SAE 45° Flare

The SAE 45° flare (SAE J512) uses a 45° included cone on a single (not double) flared tube, with the flare facing the fitting and the nut bearing on the back of the flare. It is the dominant flare in refrigeration and HVAC (copper line sets), fuel and gas appliance connections, and general soft-copper and brass plumbing. Threads are typically SAE straight (UNF) sizes designated by tube OD — for example, a 3/8" tube flare uses a 5/8"-18 nut. Brass SAE 45° fittings commonly handle several hundred PSI depending on size and tube. It is the right choice for accessible, serviceable connections on copper and aluminum tube.

Both use a 45° cone, but the seat geometry differs: the SAE 45° flare (left) seats the cone outward against the fitting nose; the inverted flare (right) tucks the cone into an inverted seat, the configuration used on automotive brake and fuel lines.

Inverted Flare

The inverted flare also uses a 45° cone, but the seat is inverted — the male fitting carries a coned nose that the flared tube seats against from the inside, and the flare nut has internal threads engaging the fitting body. Inverted flares are the standard for automotive brake lines, transmission cooler lines, and fuel lines, where the design is compact, vibration-resistant, and well suited to steel tubing and the high pressures of hydraulic brake systems. Many brake applications use a double flare (the tube is folded over on itself) for extra strength on steel lines. Inverted flare thread sizes (e.g., 3/8"-24 for 3/16" tube) differ from SAE 45° sizes, so the two are not interchangeable even though the cone angle matches.

Why 37° JIC Is Different

The other flare you will encounter is the 37° JIC (SAE J514), used throughout hydraulics. Its cone is 37°, not 45°, so a JIC fitting and an SAE 45° fitting look similar but will not seal together — the angle mismatch leaves a line contact that leaks under pressure. JIC handles much higher pressures (thousands of PSI) on steel tube and hose, which is why hydraulics standardized on it. The rule: confirm the flare angle (37° vs 45°) before assuming two flare fittings are compatible. We cover JIC in depth in our thread identification guide.

Side-by-Side Comparison Chart

Representative characteristics for brass/steel flare fittings. Confirm thread size and pressure on the data sheet.

Factor	SAE 45° Flare	Inverted Flare	37° JIC
Cone angle (included)	45°	45°	37°
Seat orientation	External nose	Inverted (internal)	External nose
Standard	SAE J512	SAE J512 (inverted)	SAE J514
Typical tube	Copper, aluminum, brass	Steel (brake/fuel)	Steel tube & hose
Typical pressure	Up to ~few hundred PSI	High (brake hydraulic)	Very high (1000s PSI)
Primary use	HVAC/R, gas, plumbing	Automotive brake/fuel	Hydraulics
Interchangeable with others?	No	No	No

Which Flare by Application

Application	Use	Why
HVAC / refrigeration line set	SAE 45°	Standard for copper R-line connections
Gas appliance connector	SAE 45°	Serviceable metal-to-metal gas seal
Automotive brake line	Inverted (double)	High-pressure steel-line standard
Transmission / fuel cooler line	Inverted	Compact, vibration-resistant on steel
Hydraulic power line	37° JIC	Rated for thousands of PSI
Soft copper / brass plumbing	SAE 45°	Re-makeable, no sealant needed
High vibration, no sealant allowed	Any flare (matched)	Metal-to-metal seal beats compression

Diagram showing a 45 degree cone meeting a 37 degree seat with a leak path highlighted

Mixing angles fails: a 45° flare on a 37° seat touches on only a thin line, leaving a leak path. Always confirm the flare angle and standard match before assembling.

Making a Good Flare Joint

A leak-free flare starts with a clean, square tube cut and a properly formed cone. Deburr the tube, slip the flare nut on before flaring (a common and frustrating mistake to skip), and form the flare with a tool matched to the angle — a 45° flaring tool for SAE/inverted, a 37° tool for JIC. The flare should be smooth, concentric, and fill the seat without cracks or splits; an over-flared cone interferes with the nut threads and an under-flared cone seats incompletely. Tighten to the fitting's torque spec — typically snug plus a fraction of a turn — never use thread sealant on the flare seat itself (the metal-to-metal cone is the seal), and for steel brake lines use a double flare for the strength the single flare lacks.

Standards & References

SAE 45° and inverted flares are defined by SAE J512 (automotive-type tube fittings); the 37° flare is SAE J514 (JIC 37° flare). Flare-tube end preparation references SAE J533. Straight thread forms follow ASME B1.1 (Unified) for the SAE/UNF nut threads. For gas-appliance connectors, confirm compliance with the applicable ANSI Z21 / CSA standard. Brake-line double flares follow SAE J1290. Always match the flare angle and standard before mating fittings.

Frequently Asked Questions

Are SAE 45° and inverted flare fittings interchangeable?

No. Although both use a 45° cone, the seat geometry and thread sizes differ — SAE 45° seats the cone on an external nose, while the inverted flare uses an inverted internal seat. Mixing them leaks. Match the specific standard, not just the angle.

Can I connect a 45° flare to a 37° JIC fitting?

No. The 8° angle difference means the cone contacts the seat on only a thin line, which leaks under pressure. Use a matched 45°-to-45° or 37°-to-37° pair, or an adapter designed to bridge the two.

Do flare fittings need thread sealant?

Not on the flare seat — the seal is the metal-to-metal contact between the cone and the seat, so sealant there can actually cause problems. Any sealant belongs only on a separate pipe-thread connection, never on the flare itself.

When should I use a double flare?

On steel brake and high-pressure lines. Folding the tube over on itself doubles the wall at the flare, giving the strength a single flare lacks on hard steel tube. Soft copper in HVAC and gas service typically uses a single SAE 45° flare.

Which flare is best for refrigeration?

The SAE 45° flare. It is the long-standing standard for copper refrigerant line connections — serviceable, re-makeable, and sealing metal-to-metal without sealant that could contaminate the system.