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 the 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

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 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".

 

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 gauge 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 it 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.

 

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.

 

You can snake Cunifer™ virtually anywhere you can snake solder, but be careful,

modern cars are very tight and sometimes you can't fit the nut or it is impossibly

tight.

 

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/45 degree) flare and

the DIN (ISO/bubble) flare.  We will refer to them as SAE or DIN flares.

 

The most common is the SAE flare.  Typically found on all American and Asian cars, but

the European style DIN 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 I.D. 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

 

 

 

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.

 

 

 

M1-3  10mm x 1.0 common European/DIN nut with DIN flare (ISO/bubble) on 3/16"/4.75mm dia. brake line

 

 

 

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.

 

What is 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 coarse thread 3/8" diameter

bolt has 16 threads per inch.  This may be called USS, UNC or NC.  The United States,

Canada and Great Britian 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 coarse 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.  A 10mm x 1.25 is a coarse pitch.  With the threads 1.25mm apart that works

out to 20.3 threads per inch.  A 1.5mm pitch is a coarser 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 until 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.  If you really need a tap or die to fit it

you can go to MSC.com to get it.  Watch out for the bastards if you need to

chase threads.

 

U.S. nuts are always used with a 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.

 

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 a 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.

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".  10mm

is .3937" in decimal measurement.  A 10mm brake line nut will measure 9.7mm or .382

of an inch.

How do I measure the threads of brake line nuts?

A thread pitch gauge is needed to measure threads accurately.  You simply compare

the teeth on the gauge to the threads on the nut.  There are two different gauges,

American and metric.

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 nut 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 is 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 coarser 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 are 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 coarser 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 coarse thread metric nuts and they are all 1.5mm

thread.  The M10-3 is 11mm od, use a 11mm or 9/16" 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.

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.