Drill a 1/8" diameter hole near the edge of the stat, away from the part that moves and allows the coolant to flow at the correct temperature. The hole prevents an air bubble from getting trapped in the cylinder head as the radiator is filled. Another sneaky trick is to leave the upper thermostat housing off as you fill the radiator slowly, and let the radiator, block, and cylinder head get full before installing the thermostat and upper hose, then topping the coolant off. That absolutely prevents air from getting trapped under the thermostat.
Jerry
