Abstract: An assembly method and tool is provided for installing a front driven gear (18) in a torpedo housing bore (24) in the lower gearcase (12) of a marine drive, without hammer blow impacting of the gear. An axial shaft (60) has a front end (62) inserted into the gear and an annular shoulder (66) for press-fitting the gear into the reduced diameter front portion (28) of the torpedo housing bore (24). A front aligner (68) is provided by an annular disc (69) having an outer circumference (70) loosley engaging the torpedo housing bore (24) and having a central opening (72) axially sliding over the axial shaft (60). A rear shaft aligner (76) has a threaded outer circumference (80) threadingly engaging a rear threaded portion (26) of the torpedo housing bore (24), and having a central axial opening (82) axially sliding over the rear end (64) of the axial shaft.