Brake line flare types: SAE double flare vs DIN / ISO bubble flare

A brake pipe ends in one of two shapes: an open funnel with its rim folded back on itself (the SAE double flare) or a rounded bulge (the bubble flare, sold as DIN or ISO). Neither seats in the other’s port, and a single-thickness flare is not used on brake pipe at all. Identify the end by its shape, not by the thread on the nut.

What do a double flare and a bubble flare look like?

A double flare is an open funnel; a bubble flare is a domed end with no funnel to look into. The American magazine MOTOR describes how each is made. For a double flare the tubing is flared outward, then folded inward over itself, so the finished end is “twice as thick because it consists of two layers of tubing”. For a bubble flare the second step leaves the end “formed into a bubble rather than an open flare”.

The instructions for a flaring kit from British tool brand Laser Tools list three forms.

Names and punches: Laser Tools, kit 4850 instructions. Appearance: MOTOR, April 2018.

What do the names SAE, DIN and ISO refer to?

The same end carries different names in the sources we opened. MOTOR calls the domed end a bubble or ISO flare and the folded funnel a double or inverted flare; Laser calls them a DIN flare and an SAE flare.

The catalogue of the British Standards Institution (BSI) lists two ISO standards: ISO 4038 for “simple flare” pipes and ISO 13486-1 for “double-flare” pipes. They are sold and we have not opened them, and we found no source stating that the DIN and ISO ends are dimensionally identical. Choose the tool by the end on the vehicle, not the name on the box.

Why will one type not seal in the other’s port?

One type will not seal in the other’s port because each flare is defined together with its own nut and seat. BSI’s summary of ISO 13486-1 covers the double flare along with its tapped holes, male fittings and seats. Brake & Front End magazine gives the reason:

“A flare is a precision device: Never take a flare for granted. To seal brake lines against 2,000 psi takes some geometry.”

Brake & Front End, 17 February 2016

The same article puts the male and female sealing surfaces “three to five degrees” apart and notes that some flares are meant to crush onto the seat. On the forum of the American magazine Grassroots Motorsports, a member with a leaking new flare was told that a bubble flare will not seal in a double flare port: “They are two entirely different flare types which do not mix together.”

Which vehicles use which flare?

MOTOR writes for the American market, where bubble flares are “usually found on import applications”. On the same forum one member calls the bubble flare “a Euro thing” and describes a metric nut on a double flare as normal for Japanese cars and many newer American ones. Others report both types on one vehicle, even on the two ends of one line.

The sources say no more; we did not open a vehicle maker’s list of makes and models. The service information and the pipe you took off decide which flare to make.

Why is a single flare never used on brake lines?

A single flare is not used on brake pipe; the end gets a double or a bubble flare. MOTOR is categorical:

“Single flares are used for some tubing joints, but never for brake lines. Brake lines must have either double (inverted) flares or bubble (ISO) flares applied to their ends, according to the application.”

MOTOR magazine, Brake Shop column, April 2018

A brake systems training module from Turkey’s Ministry of National Education likewise requires double-flared ends on steel brake pipes. MOTOR gives no reason. What follows from its description: on a single flare the nut clamps one thickness of tube wall, on a double flare two.

The word single on a tool case means something else. Laser’s kit “Creates SAE and DIN single and double flares”, yet the one-punch ends in its table are the convex flares. The manual does not say so in a sentence; the table shows that single there is a domed end made in one operation, not an unfolded funnel.

The 37° JIC / AN flare is a different end again; telling ends apart by seat angle is covered in the article on fitting and thread types.

Which flaring mistakes turn into leaks?

The sources name four mistakes: a crooked cut, burrs, a forgotten nut and a cracked flare. Brake & Front End explains why the first is hard to catch: an off-centre cut on a poorly clamped line “may look fine to the naked eye”, then leak once the fitting compresses it.

• Crooked cut. The Turkish module says an unevenly cut end can leak at the joint. It rules out saws and pinching cutters, which bend the pipe and leave swarf.
• Burrs. Laser wants the bore de-burred and the outside edge chamfered. A forum member explains: “A conventional flare seals on the internal face so that needs to be smooth with no cracks.” The nut presses on the outside, so a burr there loads that face unevenly.
• Forgotten nut. Laser gives it an exclamation mark: “ensure the tube nut is fitted to the pipe!” The Turkish module fits the nut first too.
• Cracked or lopsided flare. MOTOR on hand-tightened tools: “overtightening will likely crack the tubing at the flare”, and a cracked flare leaks.

Forum members advise tightening a weeping new flare further to bed it in. That is field opinion; the magazine’s rule is to cut it off. Why welding or brazing does not rescue a cracked pipe is covered in the article on cracked injector lines.

In what order do you identify the end in your hand?

1. Look at the end head-on. A funnel with a folded, two-layer rim is a double flare; a domed end is a bubble flare.
2. Do not trust the thread. One forum member believed every metric fitting came on a bubble-flared line. The reply: the M10 × 1.0 fitting on a Mazda Miata takes an ordinary double flare. The same member adds that a 3/8 inch fitting screws sloppily into the metric port.
3. Do not trust the pipe diameter. In Laser’s table, 4.75 mm and 3/16 inch pipe share one die set.
4. Fit new nuts. Brake & Front End prefers new fittings: old ones may be cracked, corroded or thread-damaged.

The flexible hose at the other end of the pipe is covered in the article on brake hoses and inspection.