The hole types for racks have changed over time and varies from manufacturer to manufacturer and from application to application. Rack hole type is referenced in the EIA-310 rack specification. However, there is no one hole type that is considered “standard”. This is an overview of the 19″ rack’s standard hole types and their uses.
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.
Round Unthreaded Holes
IBM was the first to widely use unthreaded holes in an OEM rack, and they used that as a standard for many years. Unthreaded, rack holes can be converted to threaded holes with the use of a clip nut. Clip nuts are not as widely used as cage nuts.
Round, unthreaded holes are popular enough that Dell made a rail specifically for this rack type called the Versa Rail. Most modern IBM and HP rails are also compatible with round, unthreaded holes. Round, unthreaded hole racks are no longer readily available since the introduction of the square hole rack.
Square Rack Holes
The square hole measures 3/8″ x 3/8″. Most racks that use square holes have steel uprights that vary from 0.085″ to 0.110″ thick.
Almost all OEM server racks are now made with square holes. Square holes allow the OEMs to design rapidly deployable rails or Rapid Rails that automatically latch into the square holes. Dell, HP and IBM all have their own versions of the Rapid or Quick Rails. These rails greatly reduce installation time.
The second reason why square hole racks are popular is that the square holes can easily be converted to a threaded hole by the installation of a cage nut. These cage nuts snap into the square holes and are available in a variety of threads.