Certainly you can crack the plastic around the set screw by over tensioning it. Proper use does not require any tension, simply seat the screw into the indent in the shaft. Perhaps the shaft was not properly aligned... pretty hard to do in the water I'd imagine.
Thanks Matt, I have looked at the MD again today and the axle was aligned correctly, by sheer luck! as you say to align the axle in the water is impossible, BUT its not that easy in the workshop either from the looks of things, bearing in mind the MD needs to be completely assembled then axle inserted [ i'm sure you understand what I mean] there are no gaps anywhere to put a pair of pliers to turn the axle in-situ, so the only option is to keep guessing the line up, then tap axle back, you could end up doing this several times before you manage to get it correctly lined up, and only then if you have a magnifying glass to look into the hole
, wouldn't it be a good idea to have a small nut on one end of the axle so it could be rotated,eliminating the guess work? also with a nut on one end the axle could only slip out one way.