Recently, I managed to crack two Sten cover windows. This allowed moisture to enter the otherwise watertight electronics compartment. Fortunately, no immediate damage occurred. There are likely several reasons for this, and they are my fault! First, it is easy to over-torque the six retaining screws when reinstalling the collimators, etc. I've been advised to only tighten them by holding onto the short end of the hex key. And, obviously, don't over tighten them. Also, and if I was aware of this I had forgotten, the cover window, gasket, bezel, and steel retaining piece are all cut differently. That is, they all have a wide side, or are slightly wider on one side when viewed edge-on. The wide side of the gasket, bezel, and steel retaining piece should be installed down. However, the cover window has to be installed wide side up or out.