Stihl string trimmers use a trademarked multipurpose cutting head attachment, according to the official Stihl website. These cutting heads, where the line is held, come in a variety of different shapes and sizes. While they are designed with slight differences, their functions are the same. Therefore, changing the line is also almost identical from model to model. Refer to your operator's manual for specific instructions on changing the line for your model. Replace the line with the same size line recommended in your operator's manual.
Switch the "On/Off" button to the off position. Let the trimmer cool down for several minutes before working on the trimmer head. Set the engine on the ground and hold the shaft in your hands.
Push the knob in the center of the head attachment down and turn it in the direction of the arrow on the head. Remove the head's cover and the center knob. Pull out the inner reel and large spring.
Remove any old line still remaining on the reel. Cut off any line that is stuck or has been fused to the reel. Wipe down the inner reel, spring and head attachment with the rag. Inspect the parts for any visible signs of damage. Replace these parts as necessary.
Cut off two equal lengths of line. For most Stihl trimmers, use two 12-foot sections of line. Refer to your operator's manual for specific lengths. Thread one line 3/4 of an inch through the bottom eyelet on the middle portion of the inner reel. There will be two plastic eyelets next to the inner flange on the reel.
Hold the short end of the line tightly against the inner reel with your thumb. Wrap the line, clockwise, around the inner reel. Make sure that each row is tight against the last and that the layers don't overlap or crisscross each other, otherwise the line won't feed out properly. Place the remaining 6 inches into the notch on the side of the upper flange.
Repeat the same process with the second string on the upper half of the inner reel. Wind it tightly and evenly; stick the remaining 6 inches into the notch on the side of the upper flange.
Hold the trimmer upside down and the head attachment. Thread the two ends of the line through the eyelets on the outer cover of the trimmer head. Push the inner reel down into the head. Set the spring in place over the reel and press down the center knob. Tighten the knob to lock the inner reel and spring in place. Wiggle the cutting head back and forth to make sure the line is secure.