iv had problems with cometic head gasket, mine kept bursting coolant hoses (over preshurising) and i had both block and head skimmed too. at 300 bhp group A is fine, i now use a wrc gasket at 2 bar boost done 8k miles now still ok. but my expansion tank bubbles when turned off, i thaught this was just the localised boiling of the coolant on the hot turbo, always ignored it and never used coolant of overheated etc.