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Aligning caliper piston 911S MFI 1970
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Ovally
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PostPosted: Thu Jan 30, 2020 11:32 am    Post subject: Aligning caliper piston 911S MFI 1970 Reply with quote

I bought new pistons and seals to overhaul the rear calipers from my 911S MFI 1970.
Does anyone know why you have to mount the pistons at an angle of 20°?

I made a 20° plate from a piece of aluminum but what is the intention of it?
The Haynes workshop manual gives no further explanation.

Image may have been reduced in size. Click image to view fullscreen.


Image may have been reduced in size. Click image to view fullscreen.


Image may have been reduced in size. Click image to view fullscreen.


Thanks in advance and regards.
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Porsche 911S MFI 1970
Mazda 929 Hardtop 1977
Mercedes SLC 180 Roadster 2018


Last edited by Ovally on Sat Feb 01, 2020 3:15 am; edited 1 time in total
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raygreenwood
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PostPosted: Fri Jan 31, 2020 11:54 am    Post subject: Re: Aligning caliper piston Reply with quote

You need to align it....so that the anti-rotation plate that fits into that slot....sits at the right angle so it can mate up with the caliper tension pin.

If you do not have those plates...you need to get them. You can possibly get them at PMB performance.

https://www.pmbperformance.com/catalog.html

You can see me installing them in this thread down about 3/4 of the way through...even though these are 914 late fronts...same function

https://www.thesamba.com/vw/forum/viewtopic.php?t=658176

Ray
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Ovally
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PostPosted: Fri Jan 31, 2020 4:26 pm    Post subject: Re: Aligning caliper piston Reply with quote

raygreenwood wrote:


You can see me installing them in this thread down about 3/4 of the way through...even though these are 914 late fronts...same function

https://www.thesamba.com/vw/forum/viewtopic.php?t=658176

Ray


Thanks for that detailed write up!
In the Porsche Genuine Parts Catalogue I found that the anti-rotation plates were indeed used in the 914 fronts.

Image may have been reduced in size. Click image to view fullscreen.


The catalogue does not show the anti-rotation plates for the 911S 1970 rear and front calipers.

Rear caliper.

Image may have been reduced in size. Click image to view fullscreen.


Meanwhile found this ATE assembly gauge https://www.stoddard.com/91135170038-sic.html.html but I made already one.
Not for the front 911 S alloy calipers which are in my car.
And this technical article https://www.pelicanparts.com/techarticles/911_caliper_rebuild/911_caliper_rebuild.htm
Don't see anything mentioned about anti-rotating plates for the 911.
So I'm a bit confused.

Regards, Armand
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raygreenwood
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PostPosted: Fri Jan 31, 2020 6:30 pm    Post subject: Re: Aligning caliper piston Reply with quote

In some cars of the era....the notched plate that fits into the piston notch was built into the back of the pads.

It would be worth calling PMB and asking if the rear needed them and if they exist. Ray
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Ovally
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PostPosted: Sat Feb 01, 2020 1:56 am    Post subject: Re: Aligning caliper piston Reply with quote

raygreenwood wrote:
In some cars of the era....the notched plate that fits into the piston notch was built into the back of the pads.

It would be worth calling PMB and asking if the rear needed them and if they exist. Ray


Thanks, have the factory Porsche rear brake pads, part number 911.352.950.03, but there are no notched plates on the back, only flat shims from one piece.
Looked at the PMB website (great site by the way) and found this "How-To" to rebuild calipers.
https://www.pmbperformance.com/brakehowto.html

This is what they say about the 20° angle in the article :
""There’s nothing I like more than exploding Internet myths. This one is the “You have to make a 20 degree angle tool to set the piston position.” You absolutely do not have to make a 20 degree angle tool. I’m not saying you don’t need to set the proper angle, you just don’t need some goofy tool to do it. The piston is already notched for this 20 degree angle you just need to know the trick on how to set the piston prior to pressing them in. It’s so much easier than you would have ever thought. Play along at home:

1. Bleeders up. Air has to escape which is why we have bleeders on our calipers and air bubbles travel… “up” so, your bleeders go up.

2. Your rotors need to travel into the face of this groove or, your groove needs to be placed “up” at a 20 degree angle facing the oncoming rotor. This notch allows your pads to angle into the oncoming rotor making for a much quieter operation. Bleeders will give you the hint on which way to place the notch.

3. That said; simple draw an imaginary line down the center of the pad cavity and place the bottom notch on this center line. As stated previously, your pistons are already cut for the 20 degree angle so, if your bottom notch is in the center of the pad cavity, your top notch is set perfectly at 20 degrees.""


PMB sells the anti-rotation plates but only included in the front caliper seal kit for the 914: https://www.pmbperformance.com/catalog/item/1925563/4747234.htm Probably 914 specific.
The kits that they (and other suppliers such as Stoddard, Pelican Parts, Rose Passion etc.) sell for the 911, front and rear, do not contain the plates.
I assume that for the 911 you have to set the pistons at the right 20° angle and no anti-rotation plates are needed but to be sure I will contact PMB.

Regards, Armand
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raygreenwood
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PostPosted: Sat Feb 01, 2020 4:45 pm    Post subject: Re: Aligning caliper piston Reply with quote

Ovally wrote:
raygreenwood wrote:
In some cars of the era....the notched plate that fits into the piston notch was built into the back of the pads.

It would be worth calling PMB and asking if the rear needed them and if they exist. Ray


Thanks, have the factory Porsche rear brake pads, part number 911.352.950.03, but there are no notched plates on the back, only flat shims from one piece.
Looked at the PMB website (great site by the way) and found this "How-To" to rebuild calipers.
https://www.pmbperformance.com/brakehowto.html

This is what they say about the 20° angle in the article :
""There’s nothing I like more than exploding Internet myths. This one is the “You have to make a 20 degree angle tool to set the piston position.” You absolutely do not have to make a 20 degree angle tool. I’m not saying you don’t need to set the proper angle, you just don’t need some goofy tool to do it. The piston is already notched for this 20 degree angle you just need to know the trick on how to set the piston prior to pressing them in. It’s so much easier than you would have ever thought. Play along at home:

1. Bleeders up. Air has to escape which is why we have bleeders on our calipers and air bubbles travel… “up” so, your bleeders go up.

2. Your rotors need to travel into the face of this groove or, your groove needs to be placed “up” at a 20 degree angle facing the oncoming rotor. This notch allows your pads to angle into the oncoming rotor making for a much quieter operation. Bleeders will give you the hint on which way to place the notch.

3. That said; simple draw an imaginary line down the center of the pad cavity and place the bottom notch on this center line. As stated previously, your pistons are already cut for the 20 degree angle so, if your bottom notch is in the center of the pad cavity, your top notch is set perfectly at 20 degrees.""


PMB sells the anti-rotation plates but only included in the front caliper seal kit for the 914: https://www.pmbperformance.com/catalog/item/1925563/4747234.htm Probably 914 specific.
The kits that they (and other suppliers such as Stoddard, Pelican Parts, Rose Passion etc.) sell for the 911, front and rear, do not contain the plates.
I assume that for the 911 you have to set the pistons at the right 20° angle and no anti-rotation plates are needed but to be sure I will contact PMB.

Regards, Armand


The plates that the 914 uses for the front....fit many cars that use the ATE caliper.....BMWs, vw 411, 412, late type 3, some volvos etc. Same plate.

The notch...as they note...just allows the pad to tilt slightly to work with the rotational grab of the rotor.

The anti-rotation plates keep tbe piston oriented in set ups where the pads might be prone to floating back when pressure is off....and rotational vibration is high enough yo possibly cause the pistons to creep around.

If the rear pads keep the pistons under a little tension it should not be an,issue. Ray
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Ovally
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PostPosted: Mon Feb 03, 2020 1:15 am    Post subject: Re: Aligning caliper piston 911S MFI 1970 Reply with quote

Mail sent to PMB.
Will definitely be back if I have an answer.
I will also contact the technical department of Porsche Classic Germany.

911S Front : 1 piece alloy (aluminium) S caliper, 48 mm pistons.
Image may have been reduced in size. Click image to view fullscreen.


Rear caliper : 38 mm pistons.

Image may have been reduced in size. Click image to view fullscreen.


Regards.
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Ovally
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PostPosted: Mon Feb 03, 2020 2:38 pm    Post subject: Re: Aligning caliper piston 911S MFI 1970 Reply with quote

Ray, received a response from PMB to this mail.

""Hello,

I own a Porsche 911S MFI 1970.
Front brakes Alloy S calipers 48mm pistons.
Rear brakes 38mm pistons.

I overhauled the calipers and set the pistons at the right angle 20°.
I wonder if the 911S 1970 must have "anti-rotation plates" like the 914?

Photo of the front sealing kit 914: I mean the plates in the middle indicated in red.
Image may have been reduced in size. Click image to view fullscreen.


Thanks in advance and regards.""


Their answer :

""They do not. Pistons don't "rotate" so we find those plates are generally unneeded in all applications.""

Regards, Armand
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PostPosted: Tue Feb 04, 2020 3:47 am    Post subject: Re: Aligning caliper piston 911S MFI 1970 Reply with quote

Porsche answer :

"It is important that the front and rear brake pistons are correctly positioned at an angle of 20°.
Due to the design, piston retaining plates do not apply to a 911S 1970."


Regards.
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Ovally
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PostPosted: Wed Feb 12, 2020 5:59 am    Post subject: Re: Aligning caliper piston 911S MFI 1970 Reply with quote

I overhauled the rear calipers of my 911S and aligned the new caliper pistons (20°).
The old rusted pistons were still at 20°.
I also looked at the position of the front caliper pistons of the alloy S calipers.
These were not rotated either.
So think that PMB is right that the pistons usually don't rotate.
I found it odd that the ATE seal kits for a 911S do not contain anti-rotation plates but apparently they are not needed.

Regards, Armand

Image may have been reduced in size. Click image to view fullscreen.

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raygreenwood
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PostPosted: Fri Feb 14, 2020 11:22 am    Post subject: Re: Aligning caliper piston 911S MFI 1970 Reply with quote

Ovally wrote:
I overhauled the rear calipers of my 911S and aligned the new caliper pistons (20°).
The old rusted pistons were still at 20°.
I also looked at the position of the front caliper pistons of the alloy S calipers.
These were not rotated either.
So think that PMB is right that the pistons usually don't rotate.
I found it odd that the ATE seal kits for a 911S do not contain anti-rotation plates but apparently they are not needed.

Regards, Armand

Image may have been reduced in size. Click image to view fullscreen.


Its interesting.....we know what the notch does....as noted its keeps the pads "tipped in" or oriented with regard to the lead edge of the pad against the direction of rotation of the rotor. It prevents grabbing/chattering.

The lack of need for the locking/anti-rotation plate....."might" be due to less pressure in the rear and/or the windage adjustment if any.

I can attest to the fact that without plates...in the front ends of 914's, 411/412 and VW type 3 cars....there is some "slow" rotation over the years when one does not use the plates.
However...I have also found that when one is using good quality pad backing shims....you can usually get away without the rotation plates because teh very fine shallow dent that the piston makes in the backing shim....pretty much keeps the piston locked against rotation.

Perhaps the same is also true of the rears on 911. Ray
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Ovally
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PostPosted: Sat Feb 15, 2020 12:42 am    Post subject: Re: Aligning caliper piston 911S MFI 1970 Reply with quote

Thank you, I can follow your explanation well and will keep an eye on the pistons during maintenance work but because both the front and rear brake pistons of the 911S were not rotated, I don't expect a problem.
Interestingly enough it keeps me busy and found this on the MG forum that also refers to Porsche / explanation.
https://www.mgexp.com/forum/mgb-and-gt-forum.1/cut-outs-on-caliper-pistons.3271848/

Image may have been reduced in size. Click image to view fullscreen.


Regards, Armand
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Porsche 911S MFI 1970
Mazda 929 Hardtop 1977
Mercedes SLC 180 Roadster 2018
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