R90S Caliper Service

[schrader7032]

I'm not sure you can get good readings with a caliper like that...it tends to ride up on surfaces and might be affected by the very outside edge of the rotor. A digital micrometer might be a better way to go.

But if you have 0.20mm thickness differences, that seems like quite a bit in my mind. I believe that these rotors are at their minimal thickness to start with...it's not recommended to turn them down like one can do with car rotors. Plus there needs to be a special machine that can surface grind it rather than turn them like you might do a brake drum, etc.

Thickness variations could cause the brake pistons to work in and out I would think.

[San Arthur]

Kurt, I also used a micrometer caliper gauge, but didn't take pictures.

What is the original and the minimum thickness for this rotors?

Thanks for your comments.

[San Arthur]

I found the information on the rotor. 0.18" or 4.572mm

[schrader7032]

Looks right...just found the same number in my Haynes manual.

[San Arthur]

Is there a way to test the master cylinder? Do they go bad easily?
I´ll disassemble for cleaning.
Thanks for your answers.

[schrader7032]

I think they tend to go bad slowly. The o-rings begin to deteriorate and allow brake fluid to slip by, resulting in loss of firmness on the handle or even the handle going all the way to the grip.

[San Arthur]

I sent my rotors to http://www.truedisk.net for evaluation. If they pass they will be resurfaced. If they don´t pass I will get a new pair from the semi floating kind.

[San Arthur]

My rotors are back from from Truedisk, thanks Thomas.


Great packing.


The look great!


I have a questions.
Is there a side I need to install the bolt holding the disk together from? Maybe in reference with the wheel rotation? Insert them from the left side?

[schrader7032]

I think I've seen someone mention which way to assemble, but I can't find anything right off. I checked Snowbum's site. Here's a picture from RealOEM:

http://www.realoem.com/bmw/enUS/showpar ... Id=34_0603

Seems to suggest that the nut goes on the inside, but I've heard some say that you can't really go by these parts fiche diagrams.

On my essentially stock /7, I have the nut visible on the outside. I did change the disk over 30 years so, I could have reversed things...that was so long ago I don't remember. But I was thinking that if the nut is visible, it might be more obvious upon inspection that it was loosening for some reason. Dunno...the nuts I have are the Nylock variety, so they shouldn't loosen up anyway.

[San Arthur]

Thanks Kurt I follow your advice.
While looking on line I found images that show bolts on both sides of the wheel.


I got my pistons from a Jaguar parts dealer at a really great price. Amazing what you can find on line.


I prepared myself to reinstall the piston in the caliper.


But I broke the o ring and learn how not to install them.


Hahahah Do not install the o ring and then try to push the piston in! O rings are expensive.

As soon as I learn how to do it I will post again. I'll use the old rings to learn and let you know.