Using a screwdriver to hole the thermostat open a little, you can more clearly see the "Hat" on the bottom. This is supposed to contact the brass washer in the block to stop flow through it the thermostat is totally open. Does it? Let's find Using a screwdriver to hold the thermostat open a little, you can more clearly see the "Hat" on the bottom. This is supposed to contact the brass washer in the block to stop flow through if the thermostat is totally open. Does it? Let's find out.