The LiftMaster 178B34 Straight Door Arm Section is the straight link in the door arm assembly that connects a trolley-style garage door opener to the top door bracket on a sectional overhead door. As a direct replacement part, it restores the correct mechanical connection between the opener and the door, supporting smooth opening and closing cycles when paired with the curved door arm. This component contains no electronics and does not affect opener frequency or voltage; it is a passive linkage used on many residential LiftMaster systems.
LiftMaster part 178B34 is commonly used with the matching curved section (178B35) on a wide range of LiftMaster, Chamberlain, and Craftsman residential chain- and belt-drive trolley openers. Always confirm fit by checking your opener’s parts diagram or manual. Installation typically involves disconnecting power, placing the door in the closed position, releasing the trolley, and attaching the straight arm to the door bracket and curved arm using the specified pins or fasteners per the opener’s instructions. After installation, set travel limits and test the safety reversal system as directed by the manufacturer. This is the straight section only; the curved door arm and any mounting hardware are available separately.