Popped into my LBS last week to buy a cheapo square-taper chainset to rebuild an old frame as an SS, and came away with a brand new FSA Gravity Light double+bash for £35 😆 Owner had forgotten he had it and has pretty much given up on the more hardcore type stock (they mostly sell road and Dutch-type bikes).

Anyway, one of the things was that he didn't have the preload bolt thing for the left-hand arm. I've found some online, but don't know what size I need. They seem to come in M12 (def. too small), M17, M18 and M19. Any idea which one I need?

Thanks.