Caliper bore pitting.

[Todd Black 88]

Ok, a question to those who reeeeeeaaly know something about this.

Why is pitting on caliper bores an issue for our style of calipers? Pistons I understand.

As far as I know, our caliper pistons Slide in and out on the caliper seal, not the piston bore, so the way I see it, as long as there is no pitting around the seal ring, it should not matter.

Is this correct? The internetz just have speculation and conjecture.

I know some caliper designs have the piston sliding along the bore, and then pitting makes sense as a bad issue.

[guibo09]

There is absolutely no issue with rust or crap on the inside bore of our pistons. Like you said, it's not a sealed surface. Just clean up and call it a day.

Pitting, scratches or damage on the outside faces below the dust boot groove result in a scrap piston.

Of course, if it's so pitted and rusted that the wall thickness and strength is compromised, then that could be an issue. But man, would have to have seen hell for that to happen.

[jalopi]

Scotchbrite & elbow grease, you're good to go

[LJ851]

Why is pitting on caliper bores an issue for our style of calipers?

The brakes are so reliable that people tend to not flush the fluid allowing moisture to build up (in the fluid) and start corroding things.

Flush your fluid every couple years on a street car.

[Todd Black 88]

Yeah, I flush my fluid yearly, but I have only owned my m3 for 8 of its 26 years, and the previous owners apparently ...............didn't.

I have just rebuilt my calipers again, this time with new stainless pistons from Korman, and noticed pitted bores, on the bottom when mounted.

[Todd Black 88]

Anyone with a definitive answer?

[guibo09]

Anyone with a definitive answer?

Seriously?

[jlevie]

The only place that rust pitting of the caliper would matter is in the o-ring groove. The seal is between that o-ring and the piston. And the piston doesn't even touch the caliper bore. As long as the piston and the o-ring groove is good the caliper is fine.

[Todd Black 88]

Awesome Jim thanks!
The Groove is fine on my calipers, and the pistons are new.

[Todd Black 88]

Rebuilt the calipers and they work great.