That hole might come in handy for a closed failure, at least you get some flow at the expense of slower warm up.

I have had a Wahler fail open and a Behr fail closed...

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

obviously, you need a thermostat. i had 1 560sl that had a stuck open thermostat the client decided was ok. about 9 months later, he lost the engine. the t/stat had cycled somewhat and got stuck closed, overheating it and pulling the threads from the block for the head bolts. good luck, chuck.

My aftermarket unit (Waller I think) failed open: the arched bracket on the top pulled loose. 50C is only 122F. I think most of the automotive engineering community would say that's really bad in a bunch of durability, economy, emissions, driveability ways. Certainly better fix it before winter.

DG

You had it right also. Strange fatigue pattern for a non moving part. the other post was showing similar signs of impending doom, good thing I replaced it before I had parts circulating