Fel Pro thermostat gaskets?

Thermostat housings can be a problem

"Back in the day" when housings were good, machined, heavy cast iron, there WAS no problem

Then "Cal China Chrome Custom" came along. And OEM alloy housings. And those silly stamped steel ones

As stated above, always check for flatness

Be careful not to overtighten so as not to warp the housing

Don't use ANY sealer that will act as a lubricant 'slime' and allow the gasket to waltz out of place

Don't overtighten

Be careful you do not overtighten

Be VERY careful not to overtighten