LiftMaster 3850 3850 Elite Series Manual - Page 6

ASSEMBLY STEP 1 Attach the Rail to the Motor Unit To avoid installation difficulties, do not run the garage door opener until instructed to do so. • Remove the two washered bolts mounted on top of motor unit. • Align rail and styrofoam over belt pulley. Cut tape from rail, chain and styrofoam. • REMOVE STYROFOAM. • Insert both washered bolts through the rail into the motor unit. Tighten bolts securely. Use only these bolts! Use of any other bolts will cause serious damage to door opener. • Position belt over the motor unit belt pulley. To avoid SERIOUS damage to opener, ONLY use bolts mounted in top of motor unit. Washered Bolt 5/16"-18x1/2" Rail Hole USE ONLY THIS TYPE AND SIZE BOLT Motor Unit Belt Pulley Rail Hole ASSEMBLY STEP 2 Set the Belt Tension • By hand, thread the spring trolley nut on the threaded shaft until it is finger tight against the trolley (Figure 1). Do not use any tools. • Insert a screwdriver tip into one of the nut ring slots and brace it firmly against the trolley (Figure 2). • Place a 7/16" open end wrench on the square end. Rotate about 1/4 turn until the spring releases and snaps the nut ring against the trolley (Figure 3). This extends the spring for optimum belt tension. Figure 1 Trolley Nut Ring Trolley Nut Ring Figure 3 BEFORE 1" (2.5 cm) AFTER RELEASE 1-1/4" (3.18 cm) 6 Trolley Threaded Shaft Spring/Trolley Nut Square End Trolley Threaded Nut Shaft Ring Nut Ring Slots Figure 2