How to find out what Metric nut is on your brake line

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.

Common Crossthread! M1-3 Metric thread nut in UA1-3 American thread union

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. It will catch one thread, making you think that it will go in, but it will destroy the threads.

How do I identify metric thread brake line nuts? (Part 1)

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.

How do I identify metric thread brake line nuts? (Part 2)

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.

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 9.8mm 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 Metric 10mm x 1.0 fine thread bolt or a standard Metric 4.75mm brake line; this will have 10mm x 1.0 thread nuts on it. You will use the bolt (or nut) for your gauge.