Monster PerfectView 400L User Manual page 19

DO NOT OVER TIGHTEN SCREWS.
6)
Once the TV mounting plate is hung on the arm
bracket, thread the remaining two M8x15 screws
and washers found in the hardware bag through
the bottom holes on the arm bracket and TV
mounting plate.
7)
Tighten down all four screws with your Allen wrench.
WARNING
15