I have found that the feed head on all of them is crap. Having to jam the head into the ground to advance the string is just a poor design. I bought an aftermarket head that you manually insert cut pieces of line into the head. It has four heavy spring loaded slots that hold the line tight. When the string wears down you just push the spring and put in another piece of string. I buy the heavy duty string and one set of strings lasts me two complete yard cuts. Bought the thing at Home Depot.