At Zeckhausen Racing, we often receive calls or emails from customers wanting to purchase caliper rebuild parts, such as dust boots or pressure seals, for their StopTech big brake kits.

Since StopTech brake kits have application-specific piston sizing, the pistons used in their calipers can vary from 28mm to 44mm in diameter in 2mm increments.  We can look up the vehicle application for our customers and let them know what their piston sizing should be.  However, it is not unusual for a brake kit from one vehicle to be installed on another, with the customer buying or fabricating adapter brackets to make it fit the different vehicle.  The bottom line is that piston sizing should be directly measured before ordering rebuild parts.

Until now, determining the size of a caliper piston has required removing pistons from the caliper.  This is because the tip of the piston, where it contacts the brake pad, is smaller in diameter than the body of the piston.  Measuring the part of the piston protruding beyond the dust boot will result in incorrect numbers.  Thus, the caliper must be removed from the car and the pistons ejected (typically with compressed air) to be measured.  This may be inconvenient, especially if a shop is being used to service the calipers.  The car would need to go in to the shop twice - once to have parts measured and then again when the parts arrive to be serviced.

However, since StopTech pistons are hollow (to reduce heat transfer into the caliper), it is possible to measure the inner diameter using a dial indicator, as shown below.  (Piston shown removed from caliper for clarity.)