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308 Front Brake Piston "Cut-Outs" - ???

Discussion in 'Technical Q&A' started by dave80gtsi, Apr 10, 2004.

  1. dave80gtsi

    dave80gtsi Formula 3
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    Nov 3, 2003
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    Dave Meredith
    Greetings to all - Whilst doing some maintenance on the front brakes of my 308, I noticed something today which I have never before seen.

    Each of the four (4) front disc brake pistons (two on each caliper, of course) has a machined "cut-out" on the perimeter edge face of the piston, i.e., that portion of the piston which comes into direct contact with the steel backing plate for the brake pads. This "cut-out" means that only a portion of the total perimeter of the piston edge pushes against the steel brake pad backer plate. Of the 360 degrees of the perimeter of the piston, I'd estimate that maybe 150 degrees has been cut out, with only the remaining 210 degrees able to push upon the pad backer plate. This "cut-out" is to a depth of maybe 2 to 3 mm. (If my description is not clear, I can likely post a digital photo, but anyone who has ever had their front calipers off should likely already understand what I am trying to describe.)

    Thinking that I might perhaps have a modified aftermarket caliper / piston, or maybe that I might be seeing the results of a braking "experiment" conducted by a previous owner, I checked my factory Parts Catalog. Sure enough, there's a drawing of these pistons which clearly indicates the cut-outs, so it was apparently meant to be.

    I've been working on auto disc brakes for close to 30 years, and I've never seen anything like this before. The presence of the cut-out means that the brake pad does not receive even face pressure from the piston, which seems like it might easily lead to uneven pad wearing. And, if it is such a good idea, why is this cut-out only on the front calipers, and not on the back?

    The only possible idea that I have come up with was that I was seeing some sort of a vent so to relieve heat / air / water / brake fluid which might collect and be trapped within the body of the piston, but that's kind of a goofy thought as they could have easily accomplished this questionable need with a much smaller cut-out area.

    Any thoughts or theories concerning the purpose of these cut-outs? Are they meant to point in a specific direction (up, down, towards the rear of the car, etc.)? And if so, why?

    Thanks - DM, puzzled in Ohio
     
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  3. chrismorse

    chrismorse Formula 3

    Feb 16, 2004
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    I recall seeing a photo in a porsche shop manual regarding the angle of the cut out, but I cannot recall either the angle or the specific purpose.
    My supposition is that the notch or cut out is there to bias the pressure on the pad toward the front of the pad to help equalize pad wear. I believe the reasoning goes something like this:As the pad exerts pressure on the disk, it heats up. The leading edge of the disk, just under the front edge of the pad is "cooler" because it has just made almost one revlution and all of the venting, slots and holes have discharged a bit of the heat. As it passes under the pad, it acquires heat, as does the pad, ergo the pad runs hotter at the rear of the contact with disc. More heat means more wear. This is where the supposition part comes in - if you notch the piston (at the rear portion of the piston), where it contacts the pad, I believe the center of pressure is shifted foreward or conversely, the pressure is reduced on the rear of the smoking pad all to help avoid taper wear and all of its sins.
    So, is the notch opposite the direction of rotation, or have I had to much beer while rapturiously washing my emerald green gtb??

    slowing for a beer, I remain,
    chris
     
  4. theunissenguido

    theunissenguido Formula 3
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    #3 theunissenguido, Apr 11, 2004
    Last edited by a moderator: Sep 7, 2017
  5. ferrarifixer

    ferrarifixer F1 Veteran
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    It's purpose is to remove/soften the leading edge that would otherwise be created and cause tapered pad wear. A leading edge would get "sucked in" to the disc, causing higher temperatures and accelerated wear.....like brake shoes in a drum system.

    The notches need to be aligned correctly....that's with the majority of contact area up high, and the forward facing edge at the forward most position....it's hard to explain but when you buy seal kits there's instructions in them.

    Be careful if fitting calipers from a 911 etc, which do fit but have the pistons installed for being in front of the axle line, and therefore the other way up....!
     
  6. chrismorse

    chrismorse Formula 3

    Feb 16, 2004
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    Thanks Phil,

    BTW Which 911 calipers will fit?
    I believe the front 308 piston is 38 mm.
    Are there any 4 piston calipers that fit the mounting ears and are designed for larger discs.
    I am looking for a less expensive front upgrade than the Brembo set up/and/or a larger caliper & disc set up that would fit in a 360 wheel.
    next hijack question, (since you have answered the original post),
    Do you know of any bigger ATE style parking brake equiped caliper that can be fitted to the 308???

    Carbs on a 512 - ibet that sounds awsome

    chris
     
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  8. ham308

    ham308 Formula Junior

    Nov 3, 2003
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    Early (1960`s) disk brakes used to have this. It is to stop squealing under light load.

    In fact I bought anti-squeal shims for my Healey that fit between the disk and the caliper piston. It was driving me crazy. They have the same effect. ie. only contact part of the circumference of the piston.
     
  9. bert308

    bert308 Formula 3
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    On my Alfa the pistons have no cut-out, the cut-out is in the brake pads, with the same effect.
     
  10. dave80gtsi

    dave80gtsi Formula 3
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  11. ferrarifixer

    ferrarifixer F1 Veteran
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    The 4 pot calipers from TR, 365/400, Daytona etc will bolt straight on to the 308 upright......but pad area remains quite small which is the real problem, and you'll need to "tee" into the double fluid inlets.

    OK, the notches need to be arranged as follows, when looking in from the gap in the dismountd caliper.......

    Right Front inner piston............steps at 3 oclock and 8 oclock
    RF outer..............steps at 9 oclock and 4 oclock
    LF Inner............9 and 4
    LF Outer..........3 and 8

    this way, the pistons push on the upper part of the pad, ie the trailing edge.

    To turn the piston if it's not correct is a recipe for disaster, but can be done very carefully. Push it out and refit it correctly is my best advise.

    The upgrade to the rear is to use an early 328 upright and handbrake mechanism, with your choice of caliper and disc. If you do this, your M/cyl capacity will be stretched and a long pedal will occur. But you really dont need to up your rears beyond steel hoses and good pads, unless you want to do 24 hour racing...........
     
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  13. theunissenguido

    theunissenguido Formula 3
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    #10 theunissenguido, Apr 13, 2004
    Last edited by a moderator: Sep 7, 2017
    Would you be willing to explain a bit clearer as to the location of the cut-outs, relative to the rotation of the disc? For example, the cut-out area of the piston sort of forms a "smile" - is the piston therefore oriented so that the center of the cut-out is down (i.e., a smiley face) or up (i.e., a frown)? Or maybe pointing towards the front or rear of the car?

    Maybe a picture / scan / drawing from a shop manual might help all of us to visualize this one a bit better?

    Thanks and Cheers - DM in Ohio[/QUOTE]


    Dave, this is written in the workshop also :
    -Using a proper square to 20° verify the position of the step on the piston and, if necessary, set it correctly (see fig.9).

    You can make this square yourself of a metal strip of 3mm thickness. The fig 9 is clear how to proceed.
    Guido
    Image Unavailable, Please Login
     
  14. dave80gtsi

    dave80gtsi Formula 3
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    Aha! So THAT's what the drawing posted earlier was meant to show!

    I've got it now - Many thanks! - DM
     

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