So, often these mufflers have a nut on the back that is just a lock, but also the back half of the muffler is threaded. So the nut on the rear needs to be loosened first(use the screw head in the front to hold the screw...dont try to unscrew from the front. After the nut is loose on the rear, loosen the screw from the front while holding the rear muffler half and it should work. If its stuck from oil you are right on with a bit of heat or light oil(or both) to loosen the gunk. sometimes I tap the screwhead on the front to seperate the halves. Hope this makes sense?