Just a brief topic and my selfish reminder memento....

To begin with, the outside dimension, specifically the height (or thickness) of the case, plays an important role. This is specified in Units (U). One Unit equals 1.75 in or 44.45 mm.

The table below shows the approximate available height for a CPU cooler in the most common sizes.

Rack height Total height (mm) Approximate maximum cooler height (mm)
1U (= 44.45) -> 29
2U (= 88.90) -> 75
3U (= 133.35) -> 110
4U (= 177.80) -> 154
5U (= 222.30) -> 195

Please note that these are only approximate values, as the actual CPU cooler clearance can be affected by the internal structure of the chassis (crossbeams, PSU placement, height of motherboard stand-offs, etc.).

To find out if the cooler of your choice fits in your rack-mounted chassis, please measure the available height above the CPU socket(s) and check it against the height specifications on the cooler’s product page.

Not only the chassis, but also other components can impact compatibility.

One important factor to consider when selecting a Noctua air cooler for rack-mounted installations is the orientation of the cooler and its airflow direction. For instance, the NH-U9 DX-4677’s airflow runs perpendicular to the long axis of the LGA4677/LGA4710 socket, making it ideal for setups where chassis exhaust fans align with this airflow. In contrast, if your system requires airflow parallel to the socket's long axis, the NH-D9 DX-4677 4U CPU cooler is a better choice.

source: https://faqs.noctua.at/en/support/s...-are-compatible-with-my-rack-mounted-chassis-

 

when rack mounting cpus, make sure you have air clearance between units and ducted fans work better for moving air laterally thru the cases(ie front to back, rather than up& down)
but still heat rises so the top of your stack will always get somewhat warmer than the bottom units. also if its really tall the top units will be closer to the sun, so be aware of the Icarus Project and their melt down..

 

Funny you mentioned this. I ran into this several years back and ended up making some makeshift mods to my case.