Per Turner Motor Sports:

BEHR has updated the expansion tank version to have better quality (~2 years ago). They simply thickened the plastic walls to help prevent cracking.

These expansion tanks last much longer.

This should be a key bit of knowledge when shopping around for a rebuild kit or individual tank.

Thanks to Keith (kayger12) for digging up the email and confirmation on this.

Quote Originally Posted by kayger12 View Post
Part numbers we have (from Turner)

Part #: 17117573781 This is the most recent expansion tank design from Hella-Behr, the OEM supplier to BMW. It replaces the original BMW part number 17137787039.
Sent from my S4 > Mason, OH