How to find out what American/British brake line nut is on your vehicle.

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.

Common crossthread! A1-3 American nut in UA16-3 metric thread union

A 3/8 x 24 tpi nut will go into a 10mm x 1.0 threaded hole, but will strip the threads when you tighten it. If you take a close look at the image on the left, you will see the threads are just barely catching.

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

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 strip the threads when you tighten it down to stop the leaking.

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

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 assortment with m/c nuts for 3/16" tube and the #4 nut assortment 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 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.

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 nut; this will have a 3/8" x 24 unf threads on it. You will use the bolt (or nut) for your gauge.