I remember from the past that I was told that BMW "recommend" that you do not spit the calipers and that if you need to do this, (because you cannot get the pistons out), then you should replace the calipers - this information came from a BMW guy I met years ago who was very knowledgeable and he said that BMW always would err on the side of complete safety - (don't split them just for the sake of it, if you need to clean or paint them etc., or even replace the pistons, keep them together)
why - because if you separate the two halves you may get distortions when tightening the new bolts which may cause the calipers to leak especially under pressure
BUT
my view has always been that you should only split the halves when it is absolutely necessary, (i.e. if it is the only way to get the pistons out) - which is the qualified advice that Snowbum has always given - if you do you should always replace the O rings etc., that should come with the kit and always inspect the surfaces to ensure that there is no damage
Saying all this, I have split (Brembo) calipers, cleaned them thoroughly, fitted new parts, reassembled them and not had any problems
- torque settings - I tightened them until they feel "right" as I have never been able to find an advised torque settings in any manual or BMW literature.
It goes without saying that you should always use new bolts
Obviously if you do this it is only sensible to keep an extra eye on them
(new calipers set are really expensive so if you are buying used just be careful and inspect them carefully - another reason which will cause most to rebuild rather than replace)
