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.