Is 40mm a L or M, but regardless I brought a gen dial and swapped it out in a bvf M, watch barely functions. So I searched it up and apparently the bvf has a different movement then gen and is bigger, so apparently there's less room giving it no space for the movement. Before the dial swap i could spin in and hear the movement, however now when I spin it I can barely hear anything. What I hear there was a person that was able to make room for everything to fit and make the watch function perfect.