calliper pistons - purpose of the cut out (with picture)

[buzzer]

I reconditioned the front callipers on an Audi TT the other day. (resisted the temptation to paint them red!!)

On the calliper pistons there are small rectangular cut outs machined on the pressure face... any idea why its there?

[paintman]

Any corresponding mark/lug on the back of genuine parts pads?

[LuS1fer]

First off, can't see the pic in work.

Most pistons are partly hollow to aid heat dispersal.

Secondly, nay pistons wind back in so have some sort of thing to permit the piston to be turned with a tool (or by a tool )

Edited by LuS1fer on Monday 21st February 11:53

[buzzer]

paintman said:

Any corresponding mark/lug on the back of genuine parts pads?

this is the pressure face, I.E. away from the pads

[buzzer]

dibbers006 said:

Does it stop them rotating?

Nope, looked for that, nothing at all in the calliper itself

[buzzer]

my two thoughts were...

used in the manufacturing process to locate the piston on the grinding machine to prevent it spinning

to allow fluid to get behind the piston when its right back in the calliper

[paintman]

buzzer said:

this is the pressure face, I.E. away from the pads

OOOPS. Senior moment

[soda]

Surely it's to allow fluid up the side of the piston to aid lubrication, the seals are much further up.

[buzzer]

paintman said:

buzzer said:

this is the pressure face, I.E. away from the pads

OOOPS. Senior moment

Have em all the time

[The Wookie]

LuS1fer said:

Most pistons are partly hollow to aid heat dispersal.

I was going to say, it could be to break 'the seal' between the rim of the pison and the back of the pad to allow air to flow in and out of it, and reduce heat transfer through the piston and back into the fluid.

[buzzer]

The Wookie said:

LuS1fer said:

Most pistons are partly hollow to aid heat dispersal.

I was going to say, it could be to break 'the seal' between the rim of the pison and the back of the pad to allow air to flow in and out of it, and reduce heat transfer through the piston and back into the fluid.

These are on the face of the piston that is INSIDE the callipers, not the face that contacts the pads

[buzzer]

soda said:

Surely it's to allow fluid up the side of the piston to aid lubrication, the seals are much further up.

But its on the back face of the piston... how could that aid lubrication?

[The Wookie]

buzzer said:

The Wookie said:

LuS1fer said:

Most pistons are partly hollow to aid heat dispersal.

I was going to say, it could be to break 'the seal' between the rim of the pison and the back of the pad to allow air to flow in and out of it, and reduce heat transfer through the piston and back into the fluid.

These are on the face of the piston that is INSIDE the callipers, not the face that contacts the pads

Ahhh, in that case I've got no idea.

Perhaps it's to allow fluid around the sides of the piston so it remains lubricated up to the seal even when the pistons are fully backed into the caliper.

It could even be a noise thing to change the resonant frequency of the piston

[soda]

buzzer said:

soda said:

Surely it's to allow fluid up the side of the piston to aid lubrication, the seals are much further up.

But its on the back face of the piston... how could that aid lubrication?

This

The Wookie said:

so it remains lubricated up to the seal even when the pistons are fully backed into the caliper.

Fluid will be present up as far as the seal, lubricating the piston as it moves in the caliper. The notch will still allow fluid access around the piston even when fully seated in the caliper.