Actually, there is a standard to which all ATX board manufacturers must comply. Now, they don't - so check before you buy.
Version 2.1 of the ATX specification states that the length of standoffs needs to be at least 0.25 inches (6.4 mm), with their cross sections fitting within 0.40 by 0.40 inches (10 mm Ă— 10 mm) square areas centered around each mounting hole on ATX motherboards.[4]
ATX Specification Version 2.1 June 2002.
3.4.2 Secondary (Bottom/Solder) Side Height Constraints
Required secondary (bottom) side motherboard height constraints for all areas (A-C, as shown in Figure 7)
are defined as follows (measured from the bottom planar surface of the motherboard PCB):
• ≤0.010" - Mounting hole standoff areas - no components. Restriction applies within 0.400" square
area centered on each required mounting hole location defined in Section 3.2. Nominal allowance is
provided only to accommodate slight reflow solder excess.
• ≤0.098" - All board circuit components (including leads) that are electrically conductive and intolerant
of direct connection to chassis ground (e.g., through-hole leads, surface mount resistors)
• ≤0.120" - Board components that are non-conductive or otherwise tolerant of direct connection to
chassis ground (e.g., connector guide/stake pins)
• ≤0.200" - Devices attached to the motherboard for the sole purpose of structural retention or stiffening
A chassis and its related elements (e.g., stiffening ribs, base pan, structural supports fasteners, etc.) must
allow ≥ 0.250" clearance to the bottom planar surface of the motherboard PCB. This does not including
mounting hole standoffs, which may extend to and contact the PCB at the mounting holes within the
prescribed 0.400"-square areas.
I would think that a case intended for gaming might allow more room for clearance of the CPU cooler - atleast if it was built since those towering screw-on CPU coolers began to be made. 8 mm does sound safer to me.
