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Technical Questions

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How do I tell what I need to replace the brake lines on my car?
What size are my brake lines?
How do I tell what the diameter of my brake lines are?
How do I tell what size (diameter) my brake lines are without fancy tools?
How do I tell how long the brake line is?
How do I tell what kind of flare is on the end of my brake line?
How do I tell what kind of flare I need if I don't have the brake lines?
What if there is nothing in the bottom of the port?
What is a 37 degree flare?
What is -3,-4, etc.?
How do I tell what kind of nut is on my brake line?
How do I identify American thread brake line nuts?
How do I identify metric thread brake line nuts?
How do I measure the diameter of brake line nuts?
How do I measure the threads of brake line nuts?
Notes for British/Girling/Dunlop users.
How do I measure the threads of brake line nuts without fancy tools?

How do I tell what I need to replace the brake lines on my car?

There are three things to sort out.
1. What size and how long is your brake line.
2. What kind of flares are on each end of line.
3. What kind of brake line nut is on each end.

Do not throw any of your old parts out until you've finished the job. Plan your job carefully, mark your parts as you remove them and make a simple diagram showing the specifications as you identify them. Please notice that we use the A/N dash sizes for tube, nut and fitting sizes.

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What size are my brake lines?

This is the easy part. There are two basic sizes of brake lines. The smaller size is 3/16"/4.75mm brake line, CNF-3; and the larger size is 1/4" CNF-4. 3/16" is found on virtually all cars and trucks made in the world. 1/4" brake line is most frequently found in short pieces between the master cylinder and ABS pump, the long run to the rear of full sized cars, trucks and hydraulic clutch lines. Cars and trucks in the 50's frequently had all 1/4" line.

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How do I tell what the diameter of my brake lines are?

If you have a caliper you can measure them, 3/16" is .188" o.d technically it is .1875", and 4.75mm is .187" which is half of a thousandth smaller. 1/4" tubing measures .250"/6.35mm. 6mm tubing is .236". Please bear in mind that a cheap caliper can easily be .010" off the mark. Production tolerance for the o.d. of brake tubing is +/- .003".

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How do I tell what size (diameter) my brake lines are without fancy tools?

Look at the master cylinder where the lines are near each other to see if there are two sizes of tubing. You can also use a 1/4" open end wrench to qauge your lines. It will fit easily onto 1/4" line and will bang back and forth on 3/16" line. A stack of three pennies is about 3/16" thick and three pennies and a nickel are around 1/4" thick. On some late model GM and most European built cars the larger line will be 6mm (.236") which is a little bit smaller than 1/4". What I always suggest is that you use 1/4" line with our nuts which are sized for 1/4" tubing. The 1/4" brake line is cheaper than 6mm brake line and will fit the flare seat, but not the 6mm nut. You can drill 6mm nuts out to 1/4" but be sure to deburr and chamfer the nut where it seats on the flare.

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How do I tell how long the brake line is?

The easiest way is to use a roll of common plumbing solder. Simply snake the solder into position on the car (or along the line if it's removed). Tuck it into the corners and lay it where you want it. Do not skimp or stretch the solder into position unless you want to come up short. Mark the solder with tape or such and pull it out, straighten it out and measure it. Your can snake Cunifer™ virtually any where you can snake solder, but be careful, modern cars are very tight and sometimes you cannot fit the nut or it is impossibly tight.

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How do I tell what kind of flare is on the end of my brake line?

This is also very easy. There are two basic types of flares used on OEM automotive brake systems throughout the world. The SAE/double (inverted/45degree) flare and the DIN/ISO bubble flare. We will refer to them as SAE or DIN flare.

The most common is the SAE flare. Typically found on all American and Asian cars, but the European style DIN flare is appearing everywhere now.

To identify the flares look at the end of the tubing. If it looks like a tiny funnel going into the ID of the tubing and the back side of the flare is at a 45 degree angle it is the SAE flare.

Never use a single flare on automotive components. The SAE/double flare standard is remarkably tolerant of production variations, but the assemblies must be the same standard.

Single flares are never allowed on steel brake lines.

A1-3 3/8 x 24 UNF common American/SAE nut with SAE flare (45 degree inverted/double) on 3/16"/4.75MM dia. brake line
M1-3 10mm x 1.0 common European/DIN nut with DIN flare (ISO/bubble) on 3/16"/4.75MM dia. brake line. If the end of the tubing looks like a button and the back side angle of the flare is 90 degrees it is the DIN flare. You must use a nut with a "lead" for the DIN flare.

This covers almost all post war production cars and parts (except Citroen).

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How do I tell what kind of flare I need if I don't have the brake lines?

Look in the port where the line goes. If the bottom looks like a volcano pointing up at you it is an SAE flare seat. If it looks funnel shaped going away from you it is a DIN flare seat.

SAE Flare (45 degree inverted/double)













DIN Flare (ISO/bubble)

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What if there is nothing in the bottom of the port?

On line locks, adjustable brake bias valves, etc. you sometimes will find no seat but a hole with a pipe thread in it. You will need an adaptor. You can adapt it to an SAE seat with our part number AD1br, but I recommend using our 37 degree adaptor, AN816-3. We do not have DIN or metric SAE adaptors.

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What is a 37 degree flare?

If you are fitting aftermarket/high performance parts you may run into 37 degree flares. The 37 degree flare was standardized as A.N. (Army/Navy) during WWII for aircraft use.AN/37 degree nuts and fittings accept single and double flares. It is acceptable practice to single flare Cunifer™ (seamless) brake line for use with AN/37 degree fittings. Never single flare steel brake line.

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What is -3,-4, etc.?

The dash size indicates the brake line diameter in sixteenth's of an inch. -3 (pronounced dash 3) is 3/16", -4 is 1/4", -5 is 5/16", and -6 is 3/8".

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How do I tell what kind of nut is on my brake line?

This is where it gets harder. The size wrench you use has nothing to do with identifying brake line nuts. Brake line nuts are more like hollow bolts and you measure the diameter of the threaded portion and the spacing of the threads. First we'll cover some basic information on thread forms.

American/British threads

In the U.S. inch system the standard fine thread 3/8" diameter bolt has 24 threads per inch (tpi). This may be called SAE, UNF, or NF. A standard course thread 3/8" diameter bolt has 16 threads per inch. This may be called USS, UNC or NC. The United States, Canada and Great Britain have shared the Unified Thread pitch system since World War II but older British post war cars often have British Standard threads. British threads are also measured in threads per inch. British threads are called BSF for fine threads and BSW or Whitworth for course threads. BS threads are not interchangeable with Unified threads.

Metric threads

Metric threads are measured by the diameter of the threaded portion and by how far apart the threads are. The common metric thread 10mm x 1.0 indicates a 10mm o.d. on the threaded portion and a thread pitch of 1.0mm, which is one thread every millimeter. Since there are 25.4 millimeters in 1 inch, that works out to 25.4 threads per inch. A 10mm x 1.25 is a course pitch. With the threads 1.25mm apart that works out to 20.3 threads per inch. A 1.5mm pitch is a courser pitch that works out to 16.9 threads per inch.

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How do I identify American thread brake line nuts?

From WWII till the mid 70's everything built in the U.S. and Canada used standard American nuts. There are only two nuts, one for 3/16" line and one for 1/4" line. There is a long style and a short style of each one. The A1-3 is for the 3/16" tubing. It measures 3/8" by 24 unf. The A2-4 is for 1/4" tubing. It measures 7/16" by 24 tpi (threads per inch). They both have 24 threads per inch (tpi).Notice that "tpi" indicates a "bastard" thread. 7/16" x 24 tpi is a bastard thread because you can't find a tap or die to fit it. Watch out for the bastards if you need to chase threads. U.S. nuts are always used with the SAE 45 degree double flare and are fully threaded. The A1-3 nut will thread very easily and loosely into a 10mm x 1.0 threaded fitting but will hopefully strip the threads when you tighten it down to stop the leaking.

In the 70's "master cylinder" nuts began appearing. Master cylinder nuts have a larger o.d. than the standard pattern. There are only three American m/c nut sizes available for the 3/16" tubing. 7/16" x 24 tpi, 1/2" x 20 unf and 9/16" x 18 unf. The A12-3 ( 7/16 x 24 tpi) is by far the most popular m/c style nut. For example, late model full size Ford trucks sometimes have them all over the place. The A15-3 (1/2" x 20 unf) and A16-3 (9/16" x 20 unf) are for Chrysler/Mopar only. For 1/4" line there are only two sizes of m/c nuts, 1/2" x 20 unf and 9/16" x 18 unf. It's becoming common practice to upgrade the brakes on older cars. If you are changing things around or working on a 70's and newer car the best thing to do is to buy the #2 nut pack with m/c nuts for 3/16" tube and the #4 nut pack with m/c nuts for 1/4" line. Towards the end of the 70's metric began appearing on American cars as outsourcing began to gather steam. The transition to metric has never fully taken place and these nuts are still commonly found all over North American made cars and trucks.

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How do I identify metric thread brake line nuts?

Now comes the hardest part. If you are working on a late model domestic car with ABS, you need a beer. We have seven different male metric thread nuts that fit 3/16" brake lines. If you ever look for a tap to chase the threads, you'll find most of them are bastard threads (if you haven't guessed by now, I really don't care for the bastards).

I'll start with the most common metric brake line nut. The M1-3 is the nut that is taking over for 3/16" tubing. It is a 10mm x 1.0 thread. German/DIN in origin, is now widely used on American cars. If you have a Bosch ABS pump, you have these nuts. It is also now appearing on Asian cars. Pay attention to the flares on both ends of the line, as this style nut is used with both SAE/double flares and DIN/bubble flares. Notice the distinctive non-threaded lead portion on the end by the flare. Do not substitute a fully threaded nut in place of a nut with a "lead". The threads will bottom out before the tube is tight giving you a false sense of accomplishment.

The standard Asian style is the M5-3, like the M1-3 is also 10mm x 1.0 thread for 3/16" tubing. It is always used with a SAE flare and is fully threaded. It looks very much like the standard A1-3 American nut. You can start the 10mm x 1.0 thread nuts in a 3/8" x 24 threaded U.S. fitting but it will immediately begin stripping the threads.

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How do I measure the diameter of brake line nuts?

A caliper is used to measure the diameter. Please bear in mind that a cheap caliper can easily be .010" off the mark. When measuring the diameter of a nut or bolt it will be slightly smaller than the stated size. 3/8 of an inch is .375" in decimal measurement. The actual measurement of the brake line nut would be .365". 10mm is .3937" in decimal measurement. A 10mm brake line nut will measure 9.7mm or .382 of an inch.

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How do I measure the threads of brake line nuts?

A thread pitch gauge is needed to measure threads accurateley. You simply compare the teeth on the gauge to the threads on the nut. There are two different gauges, American and metric.

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How do I measure the threads of brake line nuts without fancy tools?

It's not as hard as you might think. You will need a reference point. Get a standard American 3/8' fine thread bolt or a standard American 3/16" brake line; this will have a 3/8" x 24 unf thread nuts on it. You will use the bolt (or nut) for your gauge.

Identifying Brake Line Nuts for 3/16" Tubing

Lay the threads of the 3/8 unf bolt onto your nut, if it lines up perfectly your nut has 24 tpi. It is either 3/8 x 24 or 7/16 x 24. If the 3/8" bolt fits perfectly it must be the A1-3 nut (or the long style A1L-3). If it slips into the fitting and your nut is larger than the bolt it must be the A12-3 m/c nut (there are no long style m/c nuts).

If the threads are very close but do not line up perfectly and the threads on your nut are finer than 24 tpi, it must be the 1.0mm metric thread. The 1.0mm thread has 25.4 tpi and is the finest thread count found on brake line nuts. There are only three nuts with this thread )not counting long and short styles of the same nut).

If the 3/8" bolt will thread easily into the hole, it must be a 10mm hole. If your nut has a non-threaded lead on it, it must be the M1-3 (DIN) nut and may have either an SAE or DIN flare. If the nut is fully threaded it must be the M5-3 and must have an SAE/double flare. If your nut is much larger or the hole is much too large the nut must be an M6-3. This nut will have a lead and may have an SAE or DIN flare. If the threads on your nut are courser than 24 tpi, fully threaded and is about the same diameter as the 3/8" bolt it must be the 1.25mm thread with a 10mm od. This is the M7L-3 nut, sometimes one or two is used to mix things up on late model domestic cars and vintage Asian, it always has an SAE flare. If it has a non-threaded lead it must be the M4-3 nut.

Italian in origin also used to mix things up on late model domestic cars. SAE or DIN flares. If your nut is larger and courser than the 3/8" x 24 you are probably working on an American car and if it has ABS you may have identified every nut I've talked about so far on your car. You may have to buy another bolt and a six pack. If you have finished one six pack you don't really need to go get another bolt, there are only five more nuts it could be.

If your nut is fully threaded it is an American m/c nut, just buy the m/c nut assortment and be done with it. Or you can measure it with an open end wrench, its either 1/2" (A13-3) or 9/16" (A14-3). If your nut has a non-threaded lead, it must be metric. There are three different course thread metric nuts and they are all 1.5mm thread. The M10-3 is 11mm od, use an 11mm or 7/16" open end wrench to measure the nut. The m11-3 nut is 13mm od, use a 13mm or 1/2" wrench to measure the nut. The M12-3 is 14mm od, use a 14mm or 9/16" open end wrench to measure the nut.

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Notes for British/Girling/Dunlop users.

Notice that old British fittings use the obsolete SAE convex (bubble) flare with a 45 degree backside angle. Beginning in the '70's the DIN (bubble) flare with a 90 degree backside angle was adopted. You can use the P10 with either the SAE/DIN bubble flare or SAE double flare. The DIN flare will work in older fittings that had the SAE convex flare form and eliminates the problem of the flare wedging the nut open causing you to curse as you try to remove your old brake lines. If you are re-using your original brake line nuts and are having difficulties in starting the nut into the fitting, examine the end of the nut carefully for swelling.

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