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Manufacturer dimensional interchange
WorldWide Electric HDRS206 appears in the same manufacturer-published cross-reference row as Boston Gear 721 at Match installed ratio (5:1–60:1) · 2.06 in center distance.
WorldWide Electric's HD Cast Iron competitive interchange table publishes HDRS206 at 2.06 in center distance for shaft input / shaft output. Manufacturers marked by WorldWide as not dimensionally interchangeable are intentionally excluded from Replacement Intelligence. Match ratio, motor frame, shaft/bore and application ratings before ordering.
The manufacturer publishes the two models in a dimensional-interchange table or equivalent dimensional comparison.
Boston Gear 721 and WorldWide Electric HDRS206 are tied by the published source at Match installed ratio (5:1–60:1) · 2.06 in center distance.
Dimensional alignment does not by itself prove identical torque, thermal rating, service factor, ratio, accessories, or application suitability.