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by Dave Zeckhausen

Zeckhausen Racing often receives requests from customers wishing to purchase caliper rebuild parts (dust boots, pressure seals, or pistons) for their StopTech big brake kits. 

These rebuild parts may be ordered by clicking here: Caliper Rebuild Parts

Figure 1. Pressure seals and dust boots

StopTech big brake kits use application-specific piston sizing.  The pistons used for a Corvette brake kit may be completely different from those used on a BMW M3 brake kit.  Pistons range in size from 26mm to 48mm in 2mm increments.

We can look up your vehicle and let you know what the piston sizes your calipers should have.  However, it is common for a brake kit designed for one vehicle to be installed on another vehicle, with different caliper adapter brackets used to make it fit.  Some StopTech kits may have been special-ordered for a car with changes to the stock brake system, such as dual master cylinders or larger rear brakes.  In these cases, piston sizing will not match our application tables.  Thus piston sizing should be measured before ordering rebuild parts, especially if the brake kit is used and has an unknown history.

Figure 2. StopTech Caliper Piston

The tip of the piston, where it contacts the brake pad, is smaller in diameter than the body of the piston.  Measuring the portion of the piston protruding beyond the dust boot will result in incorrect numbers.  Thus, a caliper normally has to be removed and pistons ejected with compressed air so they can be measured directly.  This is inconvenient, especially if a shop is used to service the calipers.  The car must go in to the shop twice - once to have parts measured and a second time after the rebuild parts arrive.  Not to worry!  There is another way.

StopTech pistons are hollow, to reduce heat transfer into the caliper.  It is possible to measure a piston inner diameter with a dial indicator, without removing it from the caliper.  (Piston shown removed for clarity.)  With inner diameter in hand, the table below can be used to determine outer diameter.

Alternatively, dust boot diameter may be measured and piston diameter determined by subtracting 10mm.

Figure 3.  Measuring Piston Inner Diameter

(44mm piston removed from caliper for clarity)

The table below lists StopTech piston outer diameters associated with piston inner diameters and dust boot outer diameters.  This table applies to pistons used in StopTech calipers.  It may or may not be accurate for pistons in Brembo, Alcon, AP Racing, Rotora, JBT, Mov'it or OEM calipers.  For caliper brands other than StopTech, we 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 is easy to tell which size you have, even if your measurement is off by up to +/- 0.5mm.  For example, if your inner diameter measurement is in the range of 16.5mm - 17.5mm, it's a safe bet you have a 32mm (outside diameter) piston.

Piston Size

(mm)

Piston Inner Diameter

(mm)

Dust Boot Diameter

(mm)

 

 

 

 

 

 

26

11.70

36

28

13.45

38

30

15.20

40

32

16.95

42

34

18.70

44

36

20.45

46

38

22.20

48

40

23.95

50

42

25.70

52

44

27.45

54

46

29.30

56

48

30.95

58

 

Many calipers use a "staggered" piston design to assist in keeping brake pads parallel to the rotors to reduce tapered pad wear.  Thus you should record measurements of the large and small diameter pistons in your caliper.  A 6-piston caliper may have 3 different sized pistons.  Measure each piston, even if they look to be the same.  A 28mm piston is hard to distinguish visually from a 30mm piston.

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