Chamberlain C870 Owner's Manual page 7

Assembly
STEP 2 Fasten the rail to the motor unit
To avoid SERIOUS damage to garage door opener, use ONLY those bolts/fasteners mounted in the
top of the opener.
1. Insert a 1/4"-20x1-3/4" bolt into the cover protection bolt hole on the back end of the rail as
shown. Tighten securely with a 1/4"-20 lock nut. DO NOT overtighten.
2. Remove the bolts from the top of the motor unit.
3. Use the carton to support the front end of the rail.
4. Place the "U" bracket, flat side down onto the motor unit and align the bracket holes with the
bolt holes.
5. Fasten the "U" bracket with the previously removed bolts ; DO NOT use any power tools. The
use of power tools may permanently damage the garage door opener.
Bolts (Mounted in the garage door opener)
Bolt
Cover Protection Bolt Hole
Lock Nut
HARDWARE
Bolt
1/4"-20x1-3/4"
"U" Bracket
STEP 3 Install the idler pulley
1. Lay the chain/cable beside the rail, as shown. Grasp the end of the cable and pass
approximately 12" (30 cm) of cable through the window. Allow it to hang until Assembly
Step 4.
2. Remove the tape from the idler pulley. The inside center should be pre-greased. If dry,
regrease to ensure proper operation.
3. Place the idler pulley into the window as shown.
4. Insert the idler bolt from the top through the rail and pulley. Tighten with a 3/8" lock washer
and nut underneath the rail until the lock washer is compressed.
5. Rotate the pulley to be sure it spins freely.
6. Locate the rail tab. The rail tab is near the idler pulley on the front rail section. Use a flat head
screwdriver and lift the rail tab until the tab is vertical (90º).
Lock Nut
1/4"-20
Lock Washer
7
HARDWARE
Nut 3/8"
Bolt
Rail Tab
Chain/cable
Bolt
Nut
Lock Washer 3/8"
Rail
Grease Inside Pulley
Idler Pulley