Sorry, did not describe bump head. Do not know models. It is one with knurled round plastic , left hand threaded nut. The wrap direction is shown with an arrow. It has two string outlets of brass which wear out over time. The shaft is a hollow tube with a removable tension spring. I have found light greasing the shaft and the spool mating hole to be beneficial. I abuse the use of this tool to weed into dirt which causes frequent bind up and field disassembly, string loosening, and rethreading. The restring of the .090 " line when doubled is about 25 feet. The 2 stroke 31 cc unit has a click shaft which accepts hedge shears and a rotary 4 blade cutter useful on thicker branches. The last unit I bought has the "easy start" "elastic" pull cord which I hate. But I love this tool overall. Using sta bil and run dry for storage allows easy Spring startup. The Crafstman brand has changed hands, but these units were only about $125 a few years back compared to $300 up on othe brands.