Find Your Thread Size

Brake fitting threads look similar across sizes but aren’t interchangeable. A metric M10 x 1.0 and a 3/8 x 24 UNF sit almost the same size side by side, and using the wrong one strips the threads or never seals. This page is a real guide, not a guess: how to measure what you have, and a full list of every thread and line size we actually stock, pulled straight from our own catalog.

How to measure a fitting

For brake line or tube itself, measure the outer diameter directly with calipers, or compare it against a drill bit or rod of known size. Our common line sizes are 3/16″, 1/4″, 5/16″, and 3/8″.

For a threaded fitting (a banjo bolt, adaptor, or joiner), you need two numbers: the diameter across the outside of the threads, and the pitch, how far apart the threads are. A thread pitch gauge, a cheap set of comb-style gauges sold at any hardware store, is the reliable way to get pitch right.

Without one, measure the diameter with calipers or a ruler, then count how many thread peaks fall across one inch for a standard (SAE) thread, or measure the distance between two adjacent peaks in millimeters for a metric thread.

If a bolt is worn, rusted, or rounded off, measure the mating part instead where you can, a caliper or fitting it came out of reads more reliably than a damaged thread. Still not sure? Get in touch and we’ll help you figure it out.

What the designation actually means

Once you’ve measured, here’s how that lines up with the size printed on a part or listed below. In both systems the first number is the diameter across the peaks. Where they differ is the second number: metric gives you the pitch directly in millimeters, standard gives you threads per inch, so you divide.

Printable diameter gauge

A one-page PDF with every diameter we stock drawn to actual size, plus a calibration bar so you can confirm your printer didn’t scale the page down before you trust it.

Download the printable thread & fitting diameter gauge (PDF)

Standard (SAE) threads we stock

Metric threads we stock

Brake line / tube sizes we stock