I service, teach, and repair various tech items on Long...
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For anyone who is swapping out a potentially broken power supply:
If the main issue was the iMac not receiving power, once you install a new power supply it should have your iMac booting up within minutes of installation. If it doesn't boot up in a short amount of time you may have to try a logic board swap.
Arctic silver is a great thermal paste, but in my experience the swapping of the fans does not require any thermal paste. Thermal paste is usually used in between the connections of the computer's processor heat sink(s) and the processor, as well as the graphics card and the heat sinks.
I just had success using standard pliers to remove the longer screw posts, and then I used standard needle nose pliers to press down and grip the tiny screws. Please note that this method may/will cause scrapping of the aluminium cover.