930 brake piston orientation

[scphelps1]

I am still trying to track down the source of brake squealing with my front brakes. I have had the calipers rebuilt by VCI, SS brake hoses, initially factory pads, and have switched to Ferodo pads (was told they might be quieter). It's not just a slight squeal; they are LOUD! I pulled the calipers off again yesterday and the ridge on the pistons is parallel to the edge of the calipers. I have read many posts about the 911 caliper pistons and orienting them to 20 degrees. Is that so the high part of the ridge is facing the leading portion of the rotor? Or is it so the depression is facing the leading edge of the rotor? I also saw this post that indicates that the orientation for 4-piston calipers is not an issue.

caliper piston advice needed!

Is that the case?

Someone please help make the squealing stop!

[Bill Verburg]

Here is the diagram for 20&deg orientation on 2 piston calipers


The depressed portion lines up perpendicular to the rotor rotation vector.

The reason this is necessary is the outgasing resulting from the friction, this outgasing builds up as cushion of very hot gases at the trailing edge of the pad. This pic shows the pressure distribution across the face of a pad lying flat on the rotor

red is high pressure, blue low.

The depressed portion of the pad allows the pad to cock slightly, which partially equalizes the pressure across the pad face.

Modern calipers accomplish this by using differential bore sizes. The smaller leading piston exerts less force on the leading edge of the pad than the larger trailing piston does.

The 930calipers do not use differential bores and the 4 piston design precludes the rocking element of 2 piston design. It couldn't hurt to line the depressions up as on the 2 piston and may cure the noise problem.

noise is just a resonance of the caliper/pad and rotor, small changes in the geometery can have a big effect on noise. did you apply the anti noise paste?

Softer street oriented pads will resonate less than their stifer track oriented brethern.

Grit and pad mung will contribute to noise, keep 'em clean!

[scphelps1]

Thanks for the reply, Bill; I was hoping you might chime in! Please bear with my ignorance. In the first picture, is the raised edge of the piston facing the incoming rotor or is the depressed portion at the top? I also forgot to mention that these were new rotors. One thing I thought was strange when I was taking the calipers off was that the passenger side rotor seems to be able to move slightly in the hat (they are the floating type (OEM) with hats from VCI), but the driver side did not. Most of the noise seems to currently come from the passenger side.

Are the pistons able to be rotated in their bores, or would they need to come out again? Initially I had spread some caliper grease on the back of the pads, but did not when I swapped in the Ferodos. I have not used the anti noise paste. I may when I put them back together this time. Is there one you recommend?

Thanks for any insight!

[KTL]

Originally posted by scphelps1
In the first picture, is the raised edge of the piston facing the incoming rotor or is the depressed portion at the top?

Actually, the answer is yes to both of your questions. The raised edge of the piston faces the incoming vector of rotation and the depressed/shorter part of the piston is the top, left section of the piston as indicated by the red pie shape in Bill's modified picture below.

Are the pistons able to be rotated in their bores, or would they need to come out again?

Since the calipers were recently rebuilt, the pistons should be able to be rotated in place. I've been able to do this on my two piston calipers by using a pair of large snap-ring pliers that are set up for external snap rings. Any sort of tool you may have that can expand and grab within the piston recess will allow you to rotate the piston. Though i've not tried this with your turbo calipers.

Hopefully you'll be able to reduce the amount of squealing. But the floating rotors are known for being noisy. Not surprising since you've got metal-to-metal sliding occuring in a high heat area that is easily contaminated by all sorts of debris.

[scphelps1]

Thanks Kevin and Bill. That helps clear things up. I will try to rotate them this evening with some snap ring pliers.

[dyerkes]

I rebuilt my calipers on my 1986 911 Turbo this spring. The Turbo brakes, unlike other 911's from the same era, do not set at an angle of 20 degrees on the piston relative to the caliper. The Porsche workshop manual shows that the high side of the piston should be faced towards the rotation of the rotor. If you would like let me know and I will scan the page or two of the workshop manual that has the information regarding this.

Good luck,
David

[scphelps1]

If you wouldn't mind scanning that David, that might be helpful. I played with them this evening and was not successful at turning them in their bores. I then tried to push the pistons out using a small air compressor (on it's last legs!) and was only successful with getting one piston out! I was going to bring them to a shop tomorrow with compressed air, but you may have saved me the trouble. I believe they are oriented in the direction you describe.

[scphelps1]

Well, I finally had a chance to blow the pistons out (with the help (compressed air) of a new local shop, CDOC). We put the pistons back at the 20 degree orientation, as described, and Voila. No more squealing; at least not going forward. There is a slight bit in reverse, but certainly tolerable. Ask the NOVA gang if they were tolerable going forward before!!! Thanks for all the insight, yet again. This board never ceases to amaze me!