Expansion tank cap

First things first. A thermostat is a temperature controlled valve installed in the engine block under the inlet housing for the top radiator hose. The tank caps you refer to are pressure controlled valves. Your pressure tank needs a pressure rated cap, but the stock tanks require european caps - using american caps, due to the height of the tank neck, does not give proper sealing. Your expansion tank does not need a pressure cap but many, unnecessarily, have them. A non-pressure cap, as found on an overflow tank, will be what you use for that tank. Some stock pressure tanks have been modified with the proper height necks (you can have this done at any radiator shop) but you would be hard pressed to tell if that modification has been done. Most of the aftermarket SS tanks are built to properly use american caps.

Hope this helps.

Larry

Larry is correct. Back in the day, some people advocated running pressure caps on both tanks, or interchanging the large overflow tank cap with the pressure-relief cap on the small surge tank, thinking this would allow more water to circulate in the cooling system. It doessn't, but usually does no harm, either. Biggest problem is the extended length of stock tank necks on both tanks, and the very rough bottom of the necks where the cap is supposed to seal. Both allow a 14-lb cap to open at maybe 8 lbs so you never get the benefits of the advertised pressure. Worse, the caps leak, and the design of most Panteras allows the leaking water to run down the inner fender panel and straight into the right frame opening, where it rusts your suspension from the inside.