What the bore size means, why aluminium and cast iron are not interchangeable, and which details decide whether a master cylinder will fit.
A brake master cylinder converts pedal force into hydraulic pressure. Two cylinders that look the same from the outside can differ in bore, stroke, reservoir, flange and pedal interface — and on a vehicle that difference is felt directly at the pedal.
Almost every passenger car and light commercial vehicle built since the 1970s uses a tandem master cylinder: two pistons in one barrel, feeding two independent hydraulic circuits. If one circuit fails, the other still stops the vehicle. Single-circuit cylinders survive only on older vehicles, some small trucks and off-road equipment. When you order, check which one the vehicle has — the port layout and length differ.
Bore is given either in inches (fractions) or millimetres. The two are not always exact conversions, so compare the source, not the arithmetic.
| Inch | Metric (nominal) | Typical use |
|---|---|---|
| 5/8" | 15.9 mm | Small clutch master cylinders |
| 13/16" | 20.6 mm | Compact cars, some clutch masters |
| 7/8" | 22.2 mm | Very common on passenger cars |
| 15/16" | 23.8 mm | Larger cars, light vans |
| 1" | 25.4 mm | SUVs, pickups, light trucks |
| 1-1/16" | 27.0 mm | Commercial vehicles |
| 1-1/8" | 28.6 mm | Heavy pickups, vans |
A larger bore moves more fluid for the same pedal travel: less travel, more pedal effort. A smaller bore gives more travel and less effort. Fitting the wrong bore changes pedal feel and can upset the balance between front and rear circuits — it is not a tuning choice.
Both are used, and they are not interchangeable in service. Aluminium bodies are lighter and common on modern passenger cars; cast iron is used where heat and pressure are higher, typically commercial vehicles. The material also decides which brake fluid and which rebuild kit is correct. If the original is iron and you fit aluminium, mounting and thermal behaviour change.
Pedal sinking slowly under steady pressure, fluid at the pushrod end, an external leak on the barrel, or a brake fluid level that keeps dropping without a visible leak elsewhere. Rebuild kits exist for some part numbers and not for others — check before assuming a repair is possible.