Brake piston question

[gridgway]

Been working on my handbrake today - LHS not functioning at all - the spreader butterfiles had come off the brake shoes. Anyway that's all fixed. But I don't understand the pistons in the calipers. As you can see in the pic the face that pushes on the pad isn't flush, it has some indents so doesn't press evenly on the pad. Is there supposed to be some anti squeal shims in there or something? Or is it just ok as is? I thought we might have discussed this on here recently, but can't find it.

Thanks
Graham

IMG20221231145026 by graham Ridgway, on Flickr

[Lightweight_911]

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You need to set the pistons at an angle of 20 deg :



You can do it without a 'setting plate' easily - draw an imaginary line down the centre of the pad cavity and place the bottom notch on this centre line. Your pistons are already cut for the 20 degree angle so, if your bottom notch is in the centre of the pad cavity, your top notch is set perfectly at 20 degrees

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[911hillclimber]

My M type original callipers are just as Andy states, the shim is also cut to fit the angle and pad pins.

[gridgway]

Thanks Andy and Graham.

Regarding the shim and how it's cut, is it cut to fill the gap between the pad and piston where the notch is? That would mean the piston presses on the pad evenly all the way round 360 degrees.

At the moment with no shim there's the gap that the 20degree notch leaves.

Cheers

[Lightweight_911]

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The point of the 'notch/recess' is so that the pads are slightly tilted within the caliper.

I seem to remember that the main reason given was to reduce/minimise brake squeal (& possible uneven brake pad wear).

Nowadays I think that the only models stlll using the angled piston positioning plates are 914's


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[gridgway]

Great thanks Andy!

[911hillclimber]

This has been discussed several times over the years.
My 911 still has the same rear calipers which i think are original. The shim is as thick as the depth of the '20 deg recess' or slightly thinner. The shims are stainless steel and were in the M calipers when I got the car in 1988 so I think are factory spec. The shim looks just like the gauge diagram Andy has copied over (from the Haynes manual maybe), and through the 3 times I've resealed my calipers I use the shim to set the '20 degrees' of the pistons.

Not sure I would like to omit the shim as I like the idea of the piston pressing on the whole diameter to the pad, just my take on the design.
There are 2 shims per caliper on my car.

It has to be said, you will not find reference to a shim I describe for the rear M calipers on Google etc, but my shims have a slot hole through which the pad retaining pin passes accuratly.
Here is a diagram off the Design911 web site, shows NO shims!

[gridgway]

And on to the next question. I took the dust covers off and pushed the pistons out to check they weren't corroded and now can't get the fiddly dust covers on. Is there a knack?

Rubber bit first then retaining spring? Or both together?

[SeanP]

Rubber boot first then the clip. I tend the piston a little proud of the body to give me a bit of wiggle room stretching the boot

[gridgway]

Yes, that's what I'll try again tomorrow.

The boot didn't really need stretching though does it? Only a little bit over the retaining lip of the caliper. Unless I'm doing something wrong!

[911hillclimber]

IIRC, the lot sits very much close to an edge and the steel clips and cut the seal really easy.

[gridgway]

Looking at the dust covers on Design911, they look a bit different to mine if I'm not mistaken. My clips are circular in section and sit in a small groove around the dust cover. The ones on D911 look to me to be more rectangular in section...

https://www.design911shop.com/Brake-Cal ... =doofinder

[SeanP]

The clips with the flat section are original ATE items. The round ones are after market. The round ones are a sod to fit! They roll off as you are trying to fi them pulling the dust cover with it. I find that if you get the ends of the clips in first then hold them in place then work your way around you have more success.
The ATE ones are a breeze in comparison

[gridgway]

Thanks Sean, they are indeed a sod to fit! However I went back today to it afresh. I actually put some silicon lubricant on the outside surface of the boot. That worked a dream and with a bit of pushing and swearing, the clip slid down the outside into the groove a treat, phew!

It's all back together and nicely bled. Just the other three wheels to bleed and all done!

Thanks everyone for the amazing help!

[gridgway]

And just to add the boots are made by Girling. Aftermarket, maybe, but should be good quality!