One thing I have liked about 19" 4-post racks is that most server vendors have gone to toolless rails that just pop in to the square rail holes and have a locking mechanism. As I mentioned above, I am not sure if there are official depth standards, but it does seem like most servers come in short, normal, extended lengths, and many rail kits have a way to fine tune the depth (or are spring loaded).

For 2-post telecomm racks, it's assumed that mounting will only be by the rack ears and there doesn't seem to be much standardization in depth. My guess is there are some depths that vendors try to stay under, a little like how trucks(semis/lorrys) don't exceed a particular height to ensure they can pass under bridges 📦

I think it would be nice if this new 10" rack community could standardize on a couple standard depths for 4-post solutions but also the amount of room behind 2-post solutions. That would not preclude other depths, but could be a guidance for designers to ensure compatibility. This might be a "trailing edge" standard: survey what people are already doing and pick sizes that work well.

If there were to be a 4-post standard, would that imply using square holes and rail kits? Seems a little overkill possibly.

As for screws: 19" racks seem to use either 10-32, 12-24, or square holes that you can install cage nuts of whatever size you like or rails that use those holes. My personal preference has been:

2-post, ear-mounted, 2-fastener: M6 screw with large flat back head and M6 cage nuts
2-post ear-mounted, 4-fastener: M5 screw with flat head and countersink back with countersink washer
4-post: toolless if possible, otherwise same M5 as above

I really like the rack screws that come with supermicro servers which are M5x20L and come with a washer. The supermicro part number for a kit of these is MCP-410-00007-0N. The cage nuts for these are readily available in all the normal places. The heads are flat and drop to almost the level of the ears and don't catch on things and the countersink back and washers help to center them and allow you to crank down on them for 2-post applications where you need a lot of force. However, I don't know what they'd do with 3D printed plastic ears, maybe flat back would be better than countersink in that case?

I have always hated that there were multiple screw standards for 19" racks and at one point I deliberately got rid of all 10-32 and 12-24 screws/nuts and switched to only M5/M6 and that really helped. It would be nice if a 10" standard could discard that baggage entirely. But... what are existing products with tapped hole rails already using?

Uh oh!

Mini rack specification #29

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