Short answer: a thread adaptor lets you connect a metric-threaded port to an SAE-threaded line (or the reverse) without re-flaring the tube or replacing the caliper. It’s a small, inexpensive part that solves what looks like a much bigger mismatch problem.
Why the mismatch happens
Metric and SAE/inverted-flare fittings aren’t just measured differently, they use different thread standards entirely, so a metric port and an SAE line simply won’t thread together no matter how close the sizes look on a tape measure. This comes up constantly on engine swaps (a domestic caliper paired with an import master cylinder, or the reverse), on older vehicles being converted to a different brake system, and any time a replacement part ships with a different thread than the one it’s replacing.
What an adaptor actually is
A thread adaptor is a short fitting with one thread on each end, say, M10 x 1 on one side and 10 x 1.25 on the other, so the mismatched sides each get the thread they need without touching the tube itself. Some are also DPS (double pilot seal) style, using a shoulder or o-ring style seal rather than depending on flare-to-flare contact, which matters when the two sides being joined use genuinely different sealing geometries and not just different threads.
Reading an adaptor’s part description
An adaptor is always listed thread-A to thread-B, in order, for example BQ44, “10 x 1 to 12 x 1mm” converts a 10mm-diameter, 1.0mm-pitch thread to a 12mm-diameter, 1.0mm-pitch thread. Get both numbers right, not just the diameter, a 1.0mm pitch and a 1.25mm pitch on the same 10mm diameter are not interchangeable, and forcing one onto the other will strip it.
NPT is its own category
Some adaptors bridge to NPT (National Pipe Thread) rather than another metric or SAE flare thread, these show up where a fitting needs to thread into a generic pipe port rather than a caliper or hose end, and NPT is a tapered thread rather than a straight one, which is a different sealing mechanism again.
What we stock
BrakeQuip Products carries adaptors across the common metric (M10, M12), SAE inverted-flare (3/8-24, 7/16-20, 7/16-24, 9/16-18, 9/16-20, 1/2-20), and 1/8-27 NPT combinations, BQ44 and BQ260 among them. The full range is in the Thread Adaptors category. If you’re not sure which two threads you actually have, measure them or compare against the original fitting before ordering, the threads look similar across sizes but are not interchangeable.