Manufacturer functional interchange
Boston Gear F842B/F843B appears in the same manufacturer-published cross-reference row as Falk 04UCBN2(3)-A at Match installed ratio/application · Boston 800 size 840 · Foot mounted NEMA C-face.
Boston Gear publishes F842B/F843B for 800 Series size 840 in the foot mounted nema c-face 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.
Falk 04UCBN2(3)-A and Boston Gear F842B/F843B are tied by the published source at Match installed ratio/application · Boston 800 size 840 · Foot mounted NEMA C-face.
The source still requires ratings, dimensions, ratio, fit, and application conditions to be checked before ordering.