I've changed a few thermostats, the only time I changed the gasket was when they leaked. I don't know about the smart but all the gaskets I did change were non- metallic.

 

Why would you need to change the thermostat gasket to change antifreeze? You pull the lower radiator hose, and remove the plug in the bottom of the block to drain the system. You never touch the thermostat.

You should hope and pray you never have to touch it on the smart. No way to get to it besides dropping the engine and removing the intake manifold. Not a quick or easy task.

 

I suggest you stop talking to mechanics who know nothing about a smart. The smart thermostat is a plastic housing with pipes on it to which the coolant hoses in front of the engine are connected. I think the gasket is a o-ring seal. In any case, there is a note in the parts catalog that states the gasket is not available separately but only with the thermostat. The thermostat is item # 10 in the attached illustration. It is a bugger to change so there is no reason to dink with it unless it fails to operate properly.

 

Remember; most mechanics are high school drop-outs. Today, graduating seniors have trouble reading and the country is way behind the rest of the industrialized world in literacy, numeracy and problem solving; way, way behind. No wonder we are driving foreign cars and wearing foreign clothing, etc. There is little hope for change, I fear.