Clutch Delay Valve.
It is a one-way restrictor in the hydraulic line between the clutch master cylinder and clutch slave cylinder. It allows you to push the clutch down quickly (releasing the clutch) but when you release the pedal it only allows fluid to return at a predetermined rate. It can screw up your timing in terms of when you expect the clutch to engage on pedal release, causing a jerk, particularly starting off and on the 1 - 2 shift.
I have changed it in the 325, and X3. I didn't do it in the 535 as that clutch has no perceptible impact from the CDV. BMW uses a different model of CDV on various platforms (3, 5, etc). I got mine from Zeckhausen Racing, the same link that MD gave you. Great service.
The CDV doesn't change clutch life much IMO. What it does do is prevent shocks to the driveline when the original is installed. I prefer to take that responsibility on myself, in exchange for a better connection between my clutch pedal motion and actual clutch engagement.
MD gave you the lowdown, but you can't paste search results and expect them to work for another poster. You can search yourself on CDV. Here are a couple of threads to get you started.
http://www.xoutpost.com/bmw-sav-foru...-problems.html
http://www.xoutpost.com/bmw-sav-foru...-purchase.html
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2007 X3 3.0si, 6 MT, Premium, White
Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue
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