Reducer Centerline Offset and Invert Continuity
The centerline offset = (large OD - small OD) / 2 and which surface stays continuous through a size change: a concentric reducer holds the centerline but drops the invert (a dam on a gravity drain, an air pocket on a pump suction); an eccentric flat-on-bottom holds the invert (correct on a sewer); flat-on-top holds the crown (correct on a pump suction). A 6 x 4 reducer offsets 1.0625 in. Standard lay lengths per ASME B16.9 (entered by the user); the lay length is a fitting dimension, not a code minimum.
Formula and source
centerline_offset = (large_od_in - small_od_in) / 2. concentric: centerline continuous, invert rises and crown drops by the offset. eccentric-flat-bottom: invert continuous, centerline and crown drop. eccentric-flat-top: crown continuous, centerline and invert rise.
First-principles geometry; the standard lay lengths per ASME B16.9 (bundled value entered by the user; a non-standard reducer overrides), and the flat-on-bottom-holds-invert / flat-on-top-holds-crown practice, by name. Not edition-bound for the offset.
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