How to insert extension tube into the cutting head if it is removed. Pictures shown are for the hedge trimmer but the process is the same for the chainsaw and line/brush cutter heads. 

1) Insert end of tube with hole closest to end of tube into gearcase, lining up hole on tube with screw hole in gearcase. 

2) Make sure holes line up before inserting screw.

3) Use short 1/2" screw and hand screw in the screw to make sure that is goes through both holes. Do not force. If it does not go easily, then undo screw and re-check that holes are aligned.

4) Use longer 1" screw for clamp end (2nd hole) of gearcase and tighten firmly.

5) If drive shaft is sticking out from end of tube, simply wiggle end of drive shaft and push in until end of drive shaft it approx. even with end of tube.

6) Done.