GEN 1 small block Chevrolet not including 400 ci. sucks from bottom or radiator into the water pump and is pumped into the front of the block and around the cylinders were it can migrate under pressure through the holes in the head gasket and head and then travels along to collect at the front of each cylinder head to continue through the front of the intake manifold to the water neck just under the thermostat and when the thermostat is open it flows directly back to the radiator. Also ther is a coolant bypass built into the water pump, block and head on the passengers side to allow some flow in the morning for heater and defrost operation. In essence getting some warm water to areas where the heater hoses can supply coolant to the heater core. The 400 smallblock is similar but has extra steam holes drilled in the block and heads to compensate for simiesed cylinders or where the cylinders are all joined together disrupting regular block coolant flow.