Posted to the Synth-DIY mailing list by James Husted
Art Director, Web Master, Symetrix Inc/Lucid Technology

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For the 19" standard - 19.031"max , 18.312 inches hole row to hole row, 17.72 inches minimum clearance between rails.  Holes are 0.281+/-0.005 diameter, or 10-32 Class UNF-2B, 12-24 Class UNC-2B, M5X.8-6H or M6X1-6H threaded.

There is also but harder to find 24" (24.031,23.321,22.72) and 30" (30.031,29.312,28.72) standards.

All use standard height panels in what is typically called a U unit (1U 2U etc) with what is now standard RETMA hole patterns. Sometimes one can run into the older mostly obsolete Western Electric panel at surplus stores. Not much difference, just one less hole. The hole spacing on both versions repeats every 1 3/4 inches.

The holes are like this: End of rail - 1/4"- (1st)hole - 5/8" - hole - 5/8" - hole - 1/2" - (1st) hole - repeat. The WE panels dont have the middle hole (between the 5/8" jumps). So this looks like the panels should be in 1 3/4" increments, but in the standard they should be 1/32 inch less to allow clearance. The RETMA (which is now the industry standard) increments are:

• 1U = 1.719"
• 2U = 3.469"
• 3U = 5.219"
• 4U = 6.969"
• 5U = 8.719"