The string head on a McCulloch trimmer retains the bump knob, string, inner reel and spring. This head assembly feeds the string out during operation; if any of these pieces are damaged, the trimmer will not work properly. The outer spool casing and string head attachment can get worn out through excessive friction from trimming over rocky areas. These heads should be properly cared for at all times. Replace the head with only McCulloch parts, and never operate the trimmer without the string head attached.
Make sure the trimmer is off and cool. Flip the engine down and string head up.
Hold the rim of the outer spool with one hand. With the other, twist the centre bump knob clockwise to loosen it. Remove the bump knob and the bolt.
Pull the inner reel and spring from the outer spool housing. Wipe all these parts down with a rag. Change the string and any other worn out parts.
Connect the spring to the inner reel. Hold them in place over the trimmer, and screw the bump knob back on to reinstall.
The spring needs to be installed correctly for the automatic line feeder to work.