Ultimate iMac 21.5" 4K Monitor Conversion Kit

Expanding the Workspace: The iMac 21.5" 4K Monitor Conversion

After the success of my previous iMac 27" 5K conversion, I was so hooked on the display quality that I decided to add a secondary monitor to my setup. To keep the aesthetic consistent, I sourced a donor iMac 21.5" 4K (2017 model, A1418) and gave it the same "standalone monitor" treatment.

While the internal layout follows the logic of my 5K build, this smaller sibling required a few unique creative adjustments to fit everything into the tighter chassis.

Build Details:

• Controller: Specific 4K driver board suited for the A1418 panel (sourced from AliExpress) and the crossovers (also from the same marketplace) 
• Cooling Architecture: I reused the original fan tube by carefully removing the integrated heat pipes. The fan speed is managed via a dedicated DC PWM controller mounted on the rear housing for easy access.
• Power: Reused the original iMac PSU to keep the build internal and clean.
• I/O & Integration:
  • The original USB port openings were slightly widened to allow for cable passthrough.
  • To keep it looking professional, I designed and 3D-printed a custom TPU gasket/plug to seal and finish the modified I/O area.
• Controls: Unlike the 27" version, the control buttons for this build are put onto the monitor stand for quick adjustments.
• Camera, wires, etc: are similar to the previous build
• Assembly: Used fresh iMac-specific LCD adhesive strips to ensure a factory-like seal.

The result is the perfect companion to my 5K main display. Even without internal photos of this specific build, the performance speaks for itself—a color-accurate, Retina-grade secondary monitor that fits perfectly into a modern workspace.