[200q20v] Caliper piston extension distances

Michael (Prospeed Motorsports) michael at prospeedmktggrp.com
Thu Nov 8 11:02:30 EST 2001

I did some measuring this morning... Here are the measurements and
demonstration of the potential caliper/rotor issues.

Caliper (Porsche S4, 993, and 993tt (Big Red)) designed for 32 mm rotor

Rotor (Relevant Audi factory rotor)				30 mm

Gap in caliper for rotor with new pads installed		35 mm

Available Dust seal/piston travel to unseat dust seal	15 mm

Immediate piston extension with new pads
Based up 30 mm rotor 					3.5 mm

Remaining dust seal extension
with "gap slack" taken up (Pad resting against rotor)	11.5 mm

New pad thickness (PF "Z" pad)
including backing plate					15.5 mm

Backing plate thickness					5.2 mm

Maximum (new) pad material				10.3 mm

Using the 2 mm wear limit of a Porsche factory rotor applied to the Audi
rotor...If one assumes the rotor is at the wear limit and the pad
(friction material) has been 75% used, for the piston to have the pad
make contact with the face of the rotor, the piston must extend 6.85 mm,
leaving a  4.275 mm (less than 1/6 inch) margin before the dust seal
will (potential) disengage from the caliper or rip.

At this piston extension there also remains a margin of approximately 9
mm (about 3/8 inch) before the inner pressure seal seal will be

If this same formula was applied to the 996tt caliper with a 30 mm
rotor, the margin of would be only .275 mm (slightly more than 1/128
inch) In this same scenario, a margin of approximately 5 mm (about 3/16
inch) remains before the inner pressure seal seal is compromised.

While these margins appear to allow what some would feel are reasonable
and acceptable safety margins, What the number don't tell is the piston
instability when extended this far. The piston/bore/inner seal
relationship is such that a hyper extended piston does not want to
retract easily and can, during extension, have its orientation relative
to the bore canted slightly making retraction difficult if not
impossible without re-alignment.

In addition, all of this is based up a rotor worn down 2 mm. If you were
to wear the rotor down an additional 2 mm, the probability of at least a
dust boot failure would increase substantially and the margin relative
to the piston would decease in a similar fashion.

On the other hand, if you replaced the rotor (in the 32/30 scenario, not
the 34/30 scenario) after 1 mm of wear and the pads after 25% wear, I
feel you maintain reasonable and acceptable performance and safety

Michael G. Wachholz
Prospeed Motorsports

952.249.1169 office
952.475.3267 fax

400 Pond Ridge Circle
Wayzata, MN 55391

mailto:michael at prospeedmktggrp.com

More information about the 200q20v mailing list