I just cracked the locking piece of the engine oil filler cap (part # 30677911) by over turning it. I was able to pick out two big pieces but one small piece (see the pictures) is small missing, and I couldn't see it. It seems to be plastic. What should I do now?

 

Nothing much other than an oil change, hopefully it will be washed out with the oil. I wouldn't be overly concerned, a small piece of plastic is no match for the forces found in the crankcase.

 

Just drained the oil before cracking the cap. Maybe should do flush the engine?

 

I'd try hooking up a small hose to a shop vac and inserting it in the fill hole - you may get lucky and grab a piece if its still sitting up top. Otherwise as Willd noted, it may find its way down one of the oil galleys do the oil pan and you may see it come out. You said you just filled the oil so it should be clean enough to see any bits and you can throw it back in (assuming you use a clean drain pan (or buy a foil "turkey" pan to use.) The only other thing would be to remove the oil pan to see if its down there and to make sure its not caught in the pick up which could lower your oil pressure.

A true story: years ago a buddy built a high performance 454 with a very radical cam. ran the car hard one day and suddenly the oil pressure light came on. We dropped the oil pan and found that all the valve seals got mashed, found there way into the pan and clogged the pick up. Wound up redoing the heads, cleaned out the pick up and pan and the engine was good to go with no further issues.