Flange Bore Chart

The bore of a pipe flange is the inside diameter of the flange hub, and it must match the inside diameter of the pipe to which it is welded. The standard bore for ASME B16.5 flanges corresponds to Schedule 40 (standard weight) pipe dimensions. When flanges are ordered without specifying a bore, they are furnished with the standard bore listed in B16.5 Table 5. This bore is designed to align flush with Schedule 40 pipe, minimizing turbulence and erosion at the joint.

For pipe schedules other than Schedule 40, flanges must be ordered with a special bore matching the actual pipe ID. For example, a 6-inch Schedule 80 pipe has an ID of 5.761 inches versus the standard bore of 6.72 inches for Schedule 40. Using a standard bore flange on heavier wall pipe creates an internal step that can cause flow disturbance, erosion, and inaccurate flow measurement. Weld neck flanges are particularly sensitive to bore mismatch because the hub transitions directly to the pipe wall.

ASME B16.5 specifies bore tolerances of +1/16 inch for sizes up to and including NPS 12, and +3/32 inch for sizes NPS 14 and above. The minus tolerance is zero in all cases, meaning the bore may be slightly larger than nominal but never smaller. These tolerances ensure that the flange bore does not restrict flow or interfere with pipe insertion during fit-up. Custom bores outside standard tolerances require special manufacturing and should be confirmed with the flange manufacturer.