Just a little tension in the screws is needed. Screws should keep end bell seated in position until water pressure holds the end bell itself. Pre-tighten all screws evenly using only two fingers on the hex driver until endbell seats well. Then do the final torque on all screws, by tuning them 30-45 degrees more.
DO NOT EXCEED 45 degree, since deformation of threads in the plastic housing could be the result.
«Two-finger» tightning of screws. Indicator arrow shows position before final tightning
Final tightning 30-45 degrees from «two-fingers» tightning