KTM 640 Adventure rear caliper question

[AlanHolt]

I've just changed the pads on my 640, and the rear caliper piston has a plastic insert which stands about 1mm proud of the piston outer ring. The plastic is cracked, and a prod with a finger made some of it crumble away. Is this insert normal? Does the fact it's crumbling warrant a new piston? I've been working on bikes and cars for over 30 years and never seen anything plastic pushed into the end of a brake piston.

[Billy Bananahead]

I just picked this off Fowlers website.
https://www.fowlersparts.co.uk/parts/58 ... r-rear-lc4
Could it just be part of the piston sealing rings that need replacing?

[AlanHolt]

No, this is like a plastic disc that is pushed into the inner recess of the piston, so is smaller than the piston. The piston and seals are intact and in good condition.

[zimtim]

Have a look through this
http://www.ktm950.info/how/Orange%20Gar ... _rear.html
One of the pics near bottom will show you what I think you are talking about.
I recently rebuilt my rear caliper on the 950 and got new upgraded pistons with a cap on the piston.

[AlanHolt]

Thanks, that clears that up. I'll order a new piston on monday.

[zimtim]

Thanks, that clears that up. I'll order a new piston on monday.

Whilst doing your piston you may as well do the seals aswell

[~ACP~]

Yeah, I put a hole through the one in one of my pistons too, quite some time ago. I just covered it with tape so water didn't get in but it'll be pressed against the back of the pad anyway so probably not necessary. I don't think they have any critical function.

[Hugh]

Greetings,

Perhaps the 'plastic' insert has three functions, first - to prevent brake squeal, second - to prevent the pad corroding to the piston and third being that if the pad corrodes the piston and/or the caliper sticks to each other then the piston can still be retracted by deformation of the main seal

TTFN

Hugh.

[AlanHolt]

From what i've been reading, its referred to as a ventilator. As it stands slightly proud of the piston, I can only think that too much heat was being transferred from the pad to the piston, so a thin plastic spacer will stop the heat transfer.

[zimtim]

To help with heat dissipation and hopefully stop brake from going looong when riding especially off road.

One of reasons I did upgrade pistons and rebuild apart from leaking seals.