Most of the units I've seen have the plastic gears inside. The trick to making one last is to adjust the springs so that the closer is doing little work. We adjust the door without the closer attached so that a person can easily open and close the door with one hand. Your 1/4 HP unit is working overtime for a wood framed door as it's a little under powered. The one you are looking at will do a far better job and last longer. You probably can find the worm gear and replace it on your unit. I use Vaseline on the gears when installing new gears. Seems to make them last longer. Make sure the dust cover is in place over the gears too or expect short gear life from all of the grit in the air wearing them out.