Thermostat Gasket

What I discovered years ago was that after flatening the surfaces, the relief in the alloy stat house was not deep enough to receive the stat, and the stat would keep the two flat surfaces apart. It worked fine for awhile, but eventually would blow out, as the housing eventually warped downwards around it.
The cure was to flatten it again and cut the stat-cavity deeper. After I learned that, I prepped a few spares, but never needed them.
FYI,
Another thing I eventually did, after I had a solid-working system, is I installed a 7 pound cap. My hoses are now, all of them over 20 years old, some are 24yo and AFAIK the lower intake hose is an early 1970s model. IIRC the newest hose is the bypass hose, installed in around 2002.