help identify engine part

I was in the process of taking my 1993 metro engine (3 cylinder) apart for rebuild and a piece fell out of the engine and I have no idea where it came from or what it is? It is about the size of a pencil eraser and apears to be made of brass. It has a slot in it and I can see a small ball bearing inside (check ball?). Thanks for any help in advance!!

Steve

goesin front of block in the oil passage to the head, its a restictor, it just sits in the hole, if you turn engine upside down with head off will fall out

Thank you for the quick reply!! Does it go in the hole with the slot facing the cylinder head or does the slot face the base pan? Thanks again!!!

Steve