...37mm or 42mm, but 42mm seemed a little too large for such a classic watch and 37mm would have meant compromising on the size of the balance wheel, so Chris very nobly agreed to re-work the architecture to fit a 40mm case - see this post: http://AHCI.watchprosite.com/show-forumpost/fi-16/pi-6264258/ti-911958/s-0/t-ahci-christian-klings-open-version-1-of-1/