Flange Bolt-Hole Orientation

Correct bolt-hole positioning for ASME B16.5 raised-face flanges. The standard convention is two-hole symmetrical about centerlines - bolt holes straddle the vertical and horizontal axes (not aligned with them). This applies to round and rectangular bolt patterns and is the default in mating equipment unless otherwise specified.

Getting orientation wrong causes installation rework: flanges that don't mate when piping is rigged, gaskets that can't be centered, and bolt-stud lengths that don't fit because the rotation differs from drawing.

Related references: flange dimensions, pipe schedules, gasket selection.