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Manufacturer functional interchange
Boston Gear 842BF/843BF appears in the same manufacturer-published cross-reference row as David Brown M042(3)FRN at Match installed ratio/application · Boston 800 size 840 · Output flange mounted.
Boston Gear publishes 842BF/843BF for 800 Series size 840 in the output flange mounted column of its in-line helical interchange guide. Boston states the 800 Series is designed to be functionally interchangeable with listed drives while requiring verification of ratings and dimensions.
The manufacturer describes the target family as functionally interchangeable with the listed competitor model or configuration.
David Brown M042(3)FRN and Boston Gear 842BF/843BF are tied by the published source at Match installed ratio/application · Boston 800 size 840 · Output flange mounted.
The source still requires ratings, dimensions, ratio, fit, and application conditions to be checked before ordering.