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How to Measure StopTech Pistons

(without removing them from the caliper)

by Dave Zeckhausen

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.

Figure 1. Pressure seals and dust boots

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.

Figure 2. StopTech Caliper Piston

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.)

Figure 3.  Measuring Piston Inner Diameter

(44mm piston removed from caliper for clarity)

The table below lists piston outer diameters associated with piston inner diameters.  This table only applies to pistons used in StopTech calipers.  It may or may not be accurate for pistons in Brembo, Alcon, AP Racing, or OEM calipers.  If you have a caliper other than StopTech, we strongly recommend directly measuring the piston diameter before ordering replacement parts.  Caliper rebuild parts are not returnable because the wrong size was ordered - unless the error is ours.

Since pistons are available in 2mm increments, it should be easy to tell which size you have, even if your measurement is off by up to +/- 0.5mm.  For example, if you come up with a measurement in the range of 16.5mm - 17.5mm for the inner diameter, it's a safe bet you have a 32mm (outside diameter) piston.

Piston Diameter (mm)

Inner Diameter (mm)

Boot Diameter (mm)

28

13.5

38

30

15.3

40

32

17.0

42

34

18.8

44

36

20.5

46

38

22.3

48

40

24.0

50

42

25.8

52

44

27.5

54

Finally, keep in mind that most StopTech calipers use a "staggered" piston design to assist in keeping the pads parallel to the rotor and reducing tapered pad wear.  So be sure to record your measurements of both the large and small diameter pistons in your caliper.

 

To order by phone or for technical assistance:

Call (800)222-8893 and press "1" for Sales

International customers may call: 973-761-5054

Hours of operation: Monday-Friday, 10AM - 5PM  (Eastern Time)

We are located in Fairfield, New Jersey

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Last modified: 01/26/10