Most cast iron production cylinder heads came with no valve seats, but instead had the necessary angles cut into the cast iron material, forming an integral seat. With the advent of unleaded gasoline, hardened seats needed to be installed. This procedure entails locating the center of the bowl through the valve guide, then using the head shop to counter bore the area where the integral seat originally was. The new seat is then chilled in dry ice (-112 degrees F) and quickly driven into the counter bore. As a precaution, the seat is staked to prevent it from falling out.