The EIA-310-D standard (often updated to EIA/ECA-310-E) defines the universal 19-inch rack specification, standardizing mounting dimensions, hole patterns, and vertical RU spacing for IT equipment compatibility. It establishes a 1.75-inch Rack Unit and a 17.72-inch minimum opening between rails, though it does not dictate overall cabinet depth or hole type. For a detailed breakdown of the standard, visit RackSolutions EIA-310: What Does It Mean? - Rack Solutions
International equipment comes with M6 screws, but your rack has 10-32 threaded holes. EIA-310-D was written in imperial units. M6 screws (6mm diameter) are slightly larger than 10-32 (4.8mm diameter). Never force an M6 into a 10-32 hole. Use cage nuts designed for square holes or a thread adapter. The standard clarifies which hole sizes are physically exclusive. eia310d standard pdf
The standard allows round unthreaded, 10-32 threaded, and 12-24 threaded holes. It does not specify the modern "square hole" cage nut pattern. Square holes became popular through de facto industry adoption because they accept both metric (M6) and imperial (10-32) cage nuts. However, square holes are technically a later evolution (often referenced in EIA-310-E). - Rack Solutions International equipment comes with M6