Alcon Tandem Master Cylinder: 0.625" vs 0.700" Bore

The 0.625" bore (MAR7200KC180B) builds higher line pressure per stroke and delivers a firmer pedal feel, making it the right choice for lighter vehicles, smaller-piston calipers, and applications where pedal stiffness matters most. The 0.700" bore (MAR7210KC180B) displaces more fluid per stroke and suits larger-piston calipers or heavier cars where the system needs more volume to fully actuate the brakes without running out of pedal travel.

Spec Comparison

Shared platform specs sourced from the Alcon MAR72 product page. Bore-specific values confirmed via Alcon USA part numbers.

0.625" Bore (MAR7200KC180B): Who It's For

The smaller bore is the pressure-biased choice. With a smaller piston area, the same pedal force generates higher hydraulic pressure per stroke. That translates to a firmer, more immediate pedal feel and finer modulation at the top of the travel range.

This unit suits lightweight race cars paired with calipers that have a relatively modest total piston area. If your caliper spec sheets show smaller aggregate piston sizes, the 0.625" bore will pressurize that circuit quickly and keep the pedal high and confident. It is also the right fit wherever regulations require a retained diagonal split to rear brakes, since Alcon confirms this unit as suitable for Group N handbrake applications in that configuration.

Honest trade-off: The smaller bore displaces less fluid per stroke. If your calipers have large-diameter pistons or long pad knockback is a factor in your application, you may find yourself needing more pedal travel to fully seat the pads before pressure builds. Matching bore to caliper piston area is the critical step here; don't assume smaller always means better.

0.700" Bore (MAR7210KC180B): Who It's For

The larger bore moves more fluid per stroke for the same 12 mm of piston travel. That means it can fill a bigger caliper circuit faster, reaching full pad contact earlier in pedal travel before pressure rise begins. For heavier cars, larger-piston calipers, or systems where circuit volume demands are high, this is the bore to specify.

The 0.700" unit shares the KC180B body, the same forged aluminium construction, identical port threading, and the same Group N compliance as the 0.625" unit. The physical installation is identical; the only meaningful difference is how the hydraulics behave once you press the pedal. Alcon explicitly notes single-axis actuation and reduced lever effort as design features of this unit, which is useful context when sizing a pedal box and calculating mechanical advantage.

Honest trade-off: More fluid volume per stroke means lower line pressure for the same pedal input compared to the 0.625" unit. If your calipers are on the smaller side, this can make the pedal feel softer or longer than ideal. The 0.700" bore rewards a well-matched, higher total caliper piston area; it is not inherently "more" of anything without that pairing.

How to Choose

Both units share the same MAR72-family body, the same 12 mm stroke per circuit, the same forged aluminium construction, and the same port threading. The bore diameter is the only variable that matters for your build decision, and it comes down to one calculation: total caliper piston area.

The hydraulic relationship is straightforward. Line pressure equals pedal force multiplied by your pedal ratio, divided by the master cylinder piston area. A smaller bore (0.625") produces more pressure for a given input force, but moves less fluid. A larger bore (0.700") moves more fluid per stroke, but produces less pressure for the same input.

Here is the practical framework:

• Calculate your total caliper piston area. Add up the piston areas for all pistons in each circuit. Manufacturers publish bore diameters for their caliper pistons; use those to find area (πr² per piston).
• Estimate required fluid displacement. More total piston area means more fluid needed to move pads from rest to full contact. If that volume is large, the 0.700" bore closes the gap faster.
• Factor in your pedal ratio. Higher pedal ratios amplify force and partially compensate for a larger-bore master cylinder's lower inherent pressure. If your pedal box runs a high ratio, the 0.700" becomes more viable even with medium-sized calipers.
• Consider target pedal feel. Endurance and rally drivers often prefer a longer, more progressive pedal. Sprint and circuit racers tend to favor a short, firm bite. The 0.625" tilts toward firm; the 0.700" toward volume and progressive travel.

If you are building around an established caliper specification from Alcon or another manufacturer, their engineering team can confirm which bore pairs correctly with a given caliper piston area. When in doubt, consult the caliper spec sheet and work the math before ordering; swapping a master cylinder after a pedal box is installed is a straightforward job, but it is one worth avoiding.

For context on how Alcon approaches bore sizing in their pivot-mount single master cylinder range, the 0.875" Pivot-Mount Master Cylinder guide covers similar bore-selection and pedal-feel principles in a single-circuit format.