In our case, the AIO MSI CoreLiquid 240r did not fit because of the RAM. We installed the WaKü in the front. Attention! Turn the fans so that they pull and push the front fans! The leds no longer look so chic, but it worked.
Depending on what kind of mainboard you have, there are other covers in the way. A 5.5 would not fit (or very narrowly). If you absolutely want to install the AIO on top, you would have to take a very thin AIO. But if they absolutely want a 5.5 cm, they would just have to install it in the front.
mfg