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Manufacturer dimensional interchange
Boston Gear HF721 appears in the same manufacturer-published cross-reference row as Leeson 600 Series HMQ621 at Match installed ratio (5:1–100:1) · 2.06 in center distance.
Boston Gear P-8565-BG publishes HF721 as the 700 Series dimensional-interchange target for hollow output shaft — nema c flange reducer (quill input) at 2.06 in center distance. The table establishes dimensional interchange; gear ratio and application rating must be matched separately.
The manufacturer publishes the two models in a dimensional-interchange table or equivalent dimensional comparison.
Leeson 600 Series HMQ621 and Boston Gear HF721 are tied by the published source at Match installed ratio (5:1–100:1) · 2.06 in center distance.
Dimensional alignment does not by itself prove identical torque, thermal rating, service factor, ratio, accessories, or application suitability.