Threaded Holes
Threaded Rack Holes
The original standard for 19″ rack mount holes was a threaded hole. There are a variety of thread types today including 10-32, 12-24 and M6 metric. The rack uprights for a threaded hole rack are thicker than unthreaded racks to ensure the threads did not strip out.
Threaded racks work well with audio equipment and boxes that do not use rails. Most shelves are compatible with threaded racks. Most modern OEM server manufacturers only have limited success installing their rails in threaded racks. Third party rails are often required to mount the server in a threaded rack.