Two numbers decide whether a brake or clutch cylinder fits and works. Here is where to read them, how they are written, and the substitution mistakes that cost importers a container.
When a cylinder comes back as a wrong-part return, the cause is almost never the manufacturer. It is a bore or a port that was assumed rather than confirmed.
Bore diameter is a performance figure
The bore is the internal diameter of the cylinder. It sets how much fluid the piston displaces for a given pedal travel: a larger bore moves more fluid with less pressure, a smaller bore builds more pressure with less fluid. Fit the wrong bore and the pedal becomes either long and soft or short and hard — the brakes still work, but the driver feels it immediately.
It is written in millimetres or in fractional inches, and the two are not interchangeable. A 7/8 inch bore is 22.23 mm, not 22 mm. Suppliers who quote “about 22” are describing a range, not a part number.
Port thread is a fitment figure
- The port is where the brake pipe union screws in. Metric and imperial threads both exist, and they look almost identical on a photograph.
- A 10 mm and a 3/8 inch port will not swap, and forcing a union strips the thread and destroys the seal face.
- Always confirm the female thread on the cylinder, not the male thread on the existing pipe — pipes get replaced.
- Bleed screw threads are a separate specification from the inlet port. Confirm both.
The three substitutions that go wrong
First, cross-referencing by vehicle name alone: the same model name can carry two different hydraulic specifications across markets and model years. Second, pairing a wheel cylinder by bore while ignoring mounting bore distance — it fits the pipe but not the backplate. Third, swapping an aluminium unit for a cast iron one because the bore matches, which changes the weight, the heat behaviour and sometimes the pedal feel.
What to send us
If you have the old unit, a photograph with a steel rule across the bore and the port is often enough for us to confirm the part number. If you have the OE number, send it with the vehicle's year and engine code. Either way, we come back with the bore, the port thread, the mounting distance and a drawing before you commit to volume.