Chosen Solution

Is it possible to put a 21inch iMac logic board into a 27 iMac casing? Even if it doesn’t fit properly :: Will the 21inch logic board work with a 27inch iMac built-in display? - I don’t see why not.

It’s not going to have power to drive it: 21.5" iMac Intel 185W Power Supply (Late 2012) iMac Intel 21.5" (Late 2012-2019) Power Supply iMac 27" Power Supply, 300W, Late 2012 iMac Intel 27" (Late 2012-2020) Power Supply Update (07/05/2018) The 21" has a 185 watt power supply and the 27" has a 300 watt power supply.