Is it possible to put a 21 iMac logic board in 27 iMac?

Why not? The power supply seems to be fine?

Also does that mean I need to change both the power supply and the logic board? Or the power supply and the lcd?

I think @mayer was pointing out the power supplies size and connections are different.

You also have a second issue if you are thinking on connecting the 27" 5K display as the GPU in the 21.5" system wont be able to push the 5K display.

Why do you want to do this? It would be easier and cheaper just to get the 27" iMac to start with.

Ah yes I appreciate it @danj !!

Na, it's because I have a late 2012 iMac 27" that has a faulty logic board (burned out 250 Ohms Ferrite Bead Inductor) and I wanted to see if I could replace it with the logic board of a 21.5" one. But it seems as though it may be too much of a hassle.

I actually may be able to repair the 27" logic board - I am getting wonderful help in another thread, which you or @mayer may be able to help with further:: iMac on makes intro chime but screen doesn't turn on. Fans blow 100%.

I can't stress enough how wonderful you guys are with your help!! I am learning so much!!! YOU GUYS ARE THE BEST!!

Don't forget to score the answer and accept it as answered if you feel you got what you needed.

So what was the fix to your "iMac on makes intro chime but screen doesn't turn on. Fans blow 100%.

maybe ram or misconnection?

thanks for the update.