Another piston broke

I picked up all the parts from BMW yesterday but have to wait for the guide bearing shells. In the meantime started checking the ring clearance in the cylinders. Managed to replace three sets of rings. 

Cylinders 1, 3 & 4 were within spec. 

Then removed the rings from No 2. As soon as I started to remove them two large bits of piston between the ring guides fell out!

So the  rings being lined up wasn't the cause of the low compression it was the broken piston!

I've read that burned pistons are caused by a number of things:

1) Dirty or faulty fuel injector that are running way too lean.

• The injectors were tested and 2 & 3 were shown to be low compared with 1  & 4

2) Wrong heat range spark plug (too hot for the application)

• Not relevant in my case

3) Over-advanced ignition timing 

• Not relevant

4) Possibly a bad knock sensor that failed to detect detonation

• Not sure will have to check

5) On engines that are turbocharged or supercharged, too much boost pressure and/or not enough fuel can burn a piston very quickly.

• I don't think this relevant as only two pistons have been effected.