Things You'll Need
.08-inch cutting line
The Stihl FS-55 lawn trimmer comes with a drive shaft that allows the operator to interchange multiple styles of mowing heads. One of these, the Autocut 25-2, spins a nylon cutting line intended for weed eating and lawn trimming. The Autocut 25-2 comes with a tapping line advance system, which advances line whenever the head is tapped against the ground. This cutting line is wound along a removable spool, which allows you to replace the cutting line whenever necessary.
Turn the body of the trimmer upside down so that the drive shaft is facing up. Set the Autocut 25-2 onto the drive shaft. Turn the Autocut mowing head counterclockwise until it stops. Insert the stop pin into the hole on top of the gearbox to block the shaft. Tighten the mowing head counterclockwise.
Unhook the two tabs on the head of the Autocut 25-2. Slide the spool cover off the top of the mowing head. Press down on the spool and rotate it to take the spool out of the mowing head. Cut off about 33 feet of .08-inch-diameter nylon cutting line. Cut the line in half.
Insert the first line end into the hole on the top half of the spool. Wind the line following your spool's winding direction arrow. Wind the line evenly and tightly around the spool. Insert the last unwound 5-inches into the holding slot on top of the spool.
Repeat the same process for the second string, except use the opposite hole on the spool. Insert the unwound section into the opposite holding slot. Align the notches on the spool, just to the side of the eyelets on the mowing head. Press the spool down and rotate it towards the eyelets. Snap the spool cover in place. Lift up on both line ends to take them out of the holding slots.
Start the Stihl FS 55 and throttle the trigger. Hold the head one inch above the ground and tap the head against the ground to release more line from the spool. Tap the head whenever the line gets shorter than one inch; line will not advance when either side of line is less than one inches on the spool, notifying you that it's time to replace the trimmer line.