Rick, I don't know where the thread is either, but cleaning one is not too complicated. Just remove the starter from the engine. It has a O ring seal to the engine, so you may have to wiggle it out. Disconnect the battery wire, making sure that the nut is turning, not the stud.
On the bench, remove the two long skinny bolts from the case. If they don't want to turn easily, use a small Vise Grip on the bolts next to the threaded end just to break them loose. Remove the clip from the end of the gear, and note which way the gear goes on, as it will fit both ways. I like to Sharpie a couple of index lines on the ends of the case ends, altho there are some already there.
Slide the ends off the armature, making very sure you capture and note how many shim washers are on each end. These can be very thin and sometimes stick to the armature. You will probably fine a lot of black carbon dust in the brush end cap. Blow this out and also blow out the body and the armature. Chances are that the brushes are still OK, as well as the commutator bars. Just take a tooth brush and brush off the commutator, making sure to clean well between the bars. You probably will not need to undercut the bars, but take some Scotchbrite pads and shine up the commutator. Don't use steel wool as it can leave little steel threads in areas.
Examine the brush ring and the tabs on it that fit into the notches on the body. These tabs are the only thing that provide a ground path for the motor. If they are arced or dirty, clean up these areas. There is a thread in the General Information section - top post pinned, that describe how to do a alternate ground path for these motors. In my opinion, it is a good thing to do, as it seems to help the motor spin easier. Up to you, it takes a half hour to do this mod, and the first time it is a little tedious.
Now put a dab of grease in the cup on the brush end cap and slide the armature in while holding the brushes retracted. Slide on the body and add grease to the bearing on the other cap. Align your marks and install the bolts. Then replace the gear and clip. Careful on the thin O rings that go between the caps and the body. They can easily be deformed or broken.
Re-install the motor and you should notice a marked improvement.