What brand/model of pre-cat O2 sensors did you use?
Did you also replace the valve cover gasket and spark plug seals? These are "must replace" items because valve-cover removal will ruin them if they aren't already toast.
Is your car throwing any codes?
1. There's a flat gasket that fits between the vanos unit and cylinder head. It's pretty obvious and I think documented. Should be replaced when you do this job. Did you re-install the unit without a new gasket?
2. If you dropped a piston on a concrete floor, very possibly it has dented or warped and is out-of-round. Reading your description, it sounds like you didn't know about the piston with the loaded spring. It's not so hard that you can't hold the backplate down while removing or tightening the bolts.
3. When you say piston "caps" do you mean the small plastic caps with small O-rings that cover the piston bolts? If yes, I'm not clear on how one of those would be damaged.
There are various gotchas in this job but most have to do with trying to keep things well-aligned or torqued properly.
If your car isn't throwing any codes, you might still have vacuum leaks if you didn't replace the valve-cover gasket or plug seals.
Last edited by pleiades; 10-03-2012 at 10:10 AM.