[R_Builder]

Originally Posted by ossiblue

Bingo!

That would be my guess too--it's hollow, and fits snugly into the bore of the thermostat and oil port. In addition, the thermostat does not work.

I suppose it's possible for it to have fallen by accident, but to have done so, and without the "mechanic" seeing it and to fall directly into the oil port, and to line up with the bore of the piston, and to not cause a problem in seating the thermostat, almost seems too coincidental.

Actually, the t-sat can't be jammed open that way. When it opens to divert oil to the on-engine oil cooler, the insert actually moves down. It works against the prevailing oil pressure and the internal spring to do this (kind of impressive when you think about it). If someone actually jammed something in there as shown in this thread, the result would be that it was stuck CLOSED, not open.