I'm looking at purchasing a 1u (SuperMicro 815 based) server which unfortunately does not have rails - and I need to put this into a standard 19" rack [ I'm not sure of the model of rack, its whatever is at the data center).
I am aware you get universal rails (eg https://www.startech.com/en-us/server-management/unirails1ub).
What worries me is that rack rails can't be infinitely thin - and as these are universal they are not clipping in to the sides of the server. Do 1u cabinets typically have enough free space between U's them so they can work with universal rail kits or how do these things work?
In general the server chassis will be slightly lower than their stated rack unit height, to prevent that servers get wedged too tightly between the servers under and above them once mounted in a rack.
In other words: 1 U is 1.75 inch but your SuperMicro 815 based server is actually only 1.7 inch high. (According to the specs of for example this Supermicro server with the same chassis.)
So as long as the universal rail is less than 0.05 inch thick, that should work.
Alternatively: get the original sliding rail kit for that server : supermicro part MCP-290-00054-0N