Stihl weed eaters, commonly called trimmers, can use a number of different cutting heads so you can customize how the trimmer works. Some heads hold metal blades for cutting through thick brush, while others may have nylon string that advances by just tapping the trimmer on the ground. Whatever trimmer head you plan on using, there are a few universal steps that you must complete to get the old trimmer head off the machine, and the new one on.
Turn the weed eater off and unplug it if necessary. Put it on the ground or on a table with the head facing up.
Locate the small cylinder with a hole in it that extends up from the base of the head. Insert a stop pin into the hole. The stop pin would have come with the weed eater, but if you lost it any round object that will fit into the hole will work. Rotate the shaft slightly while you push the pin in, until the pin snaps into place and prevents the head from spinning.
Rotate the nut at the bottom of the mowing head counterclockwise to remove it. On some heads, you may need to use the supplied combination wrench to remove the nut.
Pull up on the head to remove it. If you are going to use a new type of mowing head, remove the stop pin and pull up on the thrust plate to remove it as well.
Put a new thrust plate onto the shaft, if needed. Rotate the thrust plate until it slides all the way down the hexagonal part of the shaft.
Place the new head on the shaft and rotate it counterclockwise until it sits all the way down on the shaft. If you used a thrust plate, you should now place the thrust washer over the shaft.
Place the cap or nut over the head and rotate it clockwise until it stops.
Insert the stop pin in the same manner that you did to remove the head.
Tighten the nut or cap as much as you can. Use the combination wrench if one came with your particular weed eater head. Remove the stop pin.