How to Measure Caliper Pistons

Brakes-Shop.com frequently receives requests for caliper rebuild parts from drivers with no idea what sizes to order.  This article describes several methods for determining the correct sizes of rebuild parts.

Caliper rebuild parts may be ordered here: Caliper Rebuild Parts

Caliper rebuild instructions may be found here: How to Rebuild Calipers

Pressure Seals and Dust Boots

A lot of well-known manufacturers like StopTech big brake kits are designed with application-specific piston sizing. This means that the piston sizes used by StopTech for a VW Golf GTi big brake kit are different than those used on a BMW M3 brake kit. Brembo, StopTech and many other manufacturers use caliper piston sizes that range in diameter from 26mm to 44mm in 2mm increments.

In some cases we can look up your vehicle and find the corresponding big brake kit to let you know the piston sizes your big brake kit calipers should have. However, calipers that have been designed for one vehicle are sometimes also installed on another vehicle, with different adapter brackets used to make them fit. In cases like this, piston sizing may not match the values in our application tables. Thus piston sizing should be measured, before ordering rebuild parts, if the big brake kit has an unknown history.

With some big brake systems, the nose of the piston, where it contacts the brake pad, tapers down to a smaller diameter than the main body. Measuring the portion protruding beyond the dust boot will result in incorrect numbers. Thus, caliper pistons typically have to be ejected, so they can be measured directly, as shown for the 28mm piston below.

Measuring Pistons Directly

This may be inconvenient, especially if a shop is used to service the calipers. In that case the vehicle would need to go into the shop twice, once to be measured and again after the rebuild parts arrive. We understand this is far from ideal so luckily there are two other ways to measure piston size without ejecting any pistons.

Measuring Piston Inner Diameter

Measure the dust boot outer diameter. Piston outer diameter may be determined by measuring the dust boot outer diameter and then subtracting 10mm. For example, a 28mm diameter piston will utilize dust boots with a 38mm outer diameter. Even if the dust boot has been vaporized from heat, you can still measure the diameter of the depression in the caliper body, where the dust boot would normally sit.

Please note that dust boots are ordered based on piston size, not dust boot diameter. Even though the dust boot shown below measures 38mm in diameter, you should order a "28mm dust boot," which can be more accurately described as a "dust boot for a 28mm piston."

To avoid confusion, a 44mm piston was removed from a caliper and used in the below image to provide some clarity.

Measuring Dust Boot Outer Diameter

Many manufacturers also have tables available that list the piston outer diameters associated with piston inner diameters and dust boot outer diameters. This example table below applies to pistons used in StopTech calipers so it may or may not be accurate for pistons in Brembo, EBC, Wilwood, AP Racing, Alcon, Rotora, JBT, Mov'it or OEM calipers. So we usually recommend directly measuring the piston diameter before ordering replacement parts.

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. Below you will find an example with a 28mm piston & dust boot removed from the caliper for clarity.

Measure the Hollow Part of the Piston

Some pistons like the ones from StopTech are hollow, to reduce heat transfer into the caliper. It is possible to measure a piston's inner diameter, without removing it from the caliper. With inner diameter in hand, the table at the bottom of this article may be used to determine piston outer diameter.

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

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.