I have my engine down to a longblock and today when I put it on the engine stand and rotated it upside down a mysterius bearing ball droppped onto the floor, its about 10 mm in diameter. The only reasonable place it could have come from is from inside the
thermostat sandwich that sits above the oil filter (oil filter is removed).

I havnt had the sandwich removed yet but will later tonight or tomorrow.

Sounds like the check ball that is in the oil filter mount.

Took the oil filter sandwich off today and yes, the little ring holder for the ball was loose and the ball fit perfectly. Thanks.