You leave the rad., cap off so all the air comes out when the thermostat opens. Hard to do with the car sitting flat. Most people say have the car at a uphill angle. I do not usually do it during break in, I do it after.
That's not what Steve and I discussed. I ALWAYS leave the upper heater hose (near the stat housing on the intake) disconnected at the heater when filling the cooling system. This leaves a NICE big "blowhole" open to the heads and block so coolant can force it's way up. You can point the hose "overboard" until you get a dribble of coolant out, then jack it up in the air to top off the system, then just hookup the hose.
IT IS POSSIBLE to airlock most engines, and it happened to me once when putting a Sig cam into my 440. I caught it in time, as I was "feeling" the heads while noticing that the top hose/ heater hoses were not getting warm.
