Another point to watch out for is trapped air in the shoulder of the radiator that is the other side to the filler neck. Gases expand far more than liquids as they heat up and so displace the coolant out of the overflow. Always ‘burp’ that air out by leaning the bike well over so that the trapped air escapes over to the filler neck side. Perhaps a two-man job if it is a heavy bike ! Don’t top up coolant to the level of the overflow pipe as you will only lose coolant as it expands. Keep it a 1/4″ or so lower.