Originally Posted by 44DCNF
Look for a slot or index hole in the collar between the gearbox and line head, that you can align and insert a punch into.That should serve to hold the shaft or gearbox from turning, while unscrewing the head. I don't know your model but most trimmers have something of that sort.
I'll post the details for anyone else who is searching. On my Husqvarna 123L, there is a hole near the gearbox. (The gearbox is the protruding nut on the side of the metal collar that holds the head to the shaft.) I put an allen wrench in the hole and rotated the head until the wrench popped in further. (the head has a hole inside that will line up with the hole your allen wrench or punch is in.) Now it will grip something internally as you twist off the head. It didn't take that much pressure to get it loose. My threads were lefties, so just turn the opposite direction to loosen than you normally would. Really simple, now that I know. By the way, I installed a Shindaiwa Speed Feed 375. It comes with an assortment of nuts and bolts. The correct nut was already installed, so you don't have to mess with any other hardware to install on the 123L, except the plastic spacer included.