Having recently had my trimmer head jam up on me, i took it apart to find one of the con rods had started to break up where the bearings rub on the spur gear cam, and as a result loose parts had quickly stopped play!
I took everything out and removed as much of the old grease and bits of scrap bearing as possible, see below.
As mine had damaged parts i just pulled it all out but be aware that if you are reusing bits then the bearings in the larger head end of the con rods are actually loose when removed from the spur gear cam, so care is needed to avoid dislodging any. you will see in a later photo how the spur gear has been worn by said bearings coming loose and shifting position and wearing grooves into it.
Above shows the case cover with gasket attached, the new spur gear and pair of con rods, and a strip of metal ive no idea what its really called, but it goes between the top con rod and case cover, to stop the con rod rubbing on the gasket.
Top con rod now in position, first though you have to get the blades in place, with the bottom one located into the hole in the con rod smaller end, then drop the top con rod in, again locating the end onto the blade, you may have to move the blades by hand to line everything up, note the white plastic grease seal/clamp screwed in position over the blades. Also in this pic look closely at the larger end of the con rod and you can clearly see the bearings, they can easily be dislodged if care is not taken when removing/installing the con rod onto the spur gear cam.
Metal spacer/rubbing strip in place, not shown in this pic but, squeeze in more grease around the edges and around the smaller drive gear/pinion on the left of the shot. replace cover, and that's it, simples! Run the unit and see if any grease oozes out where the blades enter the head, if it does you've probably got enough in there, if not use the grease point and add some more to be on the safe side. I would certainly recommend adding more grease after the first use with the new components, as the grease will obviously be flung about around the edges of the head, to where there would normally already be grease sitting, thus leaving less on the moving parts.
Hope that's of help to some one