IBM REDPAPER 520Q Technical Overview And Introduction page 36

Figure 1-5 Top view of non-IBM rack specification dimensions
The vertical distance between the mounting holes must consist of sets of three holes
spaced (from bottom to top) 15.9 mm (0.625 in.), 15.9 mm (0.625 in.), and 12.67 mm
(0.5 in.) on center, making each three-hole set of vertical hole spacing 44.45 mm (1.75 in.)
apart on center. Rail-mounting holes must be 7.1 mm + 0.1 mm (0.28 in. + 0.004 in.) in
diameter. See Figure 1-6 and Figure 1-7 on page 23 for the top and bottom front
specification dimensions.
Figure 1-6 Rack specification dimensions, top front view
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IBM System p5 520 and 520Q Technical Overview and Introduction