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How to Measure StopTech Pistons
(without removing them from the caliper)
by Dave Zeckhausen |
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At Zeckhausen Racing,
we often receive calls or emails from customers wanting to purchase caliper
rebuild parts for their StopTech big brake kits.
These parts may be
ordered on-line:
Caliper Rebuild Parts
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Figure 1. Pressure
seals and dust boots |
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Since StopTech
brake kits have application-specific piston sizing, the piston diameters used can vary from 26mm to 44mm 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 designed for one vehicle to
be installed on another, with caliper adapter
brackets to make it fit the different vehicle. Or the kit could have been
special-ordered for a car with changes to the stock braking system, such as dual
master cylinders or larger rear brakes. In such cases, the piston sizing
will not match our application tables. For these reasons, piston sizing should be directly measured before ordering rebuild parts,
especially if the brake kit was purchased used and you are not certain of its
history.
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Figure 2.
StopTech Caliper Piston |
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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 needed to 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
a second time when the rebuild parts arrive.
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 without removing the pistons from the caliper. (Piston shown removed for
clarity.)
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Figure 3. Measuring Piston Inner Diameter
(44mm piston removed from caliper for clarity) |
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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, Rotora, JBT, Mov'it or OEM calipers. If
you have a caliper 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 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. |
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Piston Diameter
(mm) |
Inner Diameter
(mm) |
Boot Diameter
(mm) |
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26 |
11.7 |
3 6 |
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28 |
13.5 |
38 |
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30 |
15.3 |
40 |
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32 |
17.0 |
42 |
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34 |
18.8 |
44 |
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36 |
20.5 |
46 |
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38 |
22.3 |
48 |
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40 |
24.0 |
50 |
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42 |
25.8 |
52 |
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44 |
27.5 |
54 |
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Finally, most
StopTech calipers use a "staggered" piston design to assist in keeping the
brake pads
parallel to the rotor and reducing tapered pad wear. Thus you should record
measurements of both the large and small diameter pistons in your caliper.
A 6-piston caliper may have as many as 3 different sizes of pistons.
Measure each one, even if they look to be the same size. A 28mm piston is
hard to distinguish visually from a 30mm piston. |
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