The LiftMaster 178B35 Curved Door Arm Section is an OEM replacement component used on trolley‑style LiftMaster, Chamberlain, and many Craftsman residential garage door openers. As the curved half of the two‑piece door arm assembly, it connects the opener’s trolley to the door bracket and creates the correct travel arc for smooth opening and closing of sectional overhead doors. Designed for chain- and belt‑drive systems that use a straight + curved arm, the 178B35 helps restore proper door movement and alignment when the original arm is bent, worn, or missing.
Installation typically involves transferring the existing clevis and cotter pins, attaching the curved section to the straight arm and the door bracket using the pre‑drilled adjustment holes, and then resetting the opener’s travel and force limits per the manufacturer’s instructions. For safety, disconnect power, engage the manual release, and ensure the door is balanced before work; spring tension and an unbalanced door can be hazardous. Verify compatibility by checking your opener’s parts diagram or owner’s manual for part number 178B35.