Help please, coolant concerns

[steelnads]

Hi folks, My griff 500 engine has just undergone a rebuild following cracked heads. These were repaired but I am concerned because the coolant level drops in the expansion tank by 2 or 3 mm every 100 miles and also there are a few bubbles visible when peering in. There are no obvious coolant leaks.
Do you think I have a problem?

Thanks Steelnads

[steelnads]

(How the hell do you measure it so accuratly anyway).
its easy when your paranoid
Thanks for the comments i will get it checked under warranty.