Why is the end of the shaft, and the oil, overheating?
After running the weed whacker for a few minutes the end of the shaft becomes so hot that it will melt the cutting head, you can also hear the oil boiling when you turn the engine off.
My guess is that the bearing or bushing centering the rotating shaft that spins the cutting head has seized or is damaged beyond repair. It may be easy to remove the cutting head and possibly access the failed bearing/bushing without too many specialized tools.
Make sure there is no dirt and junk around the engine unit. The shaft may need to be re-lubricated with a high heat lube.
If the unit is a 2-cycle engine, make sure you have the correct fuel-oil mixture in the tank. Having no oil in the fuel mixture will ruin the unit, and possibly lead to melting parts.
If the unit is 4-cycle (not common for a weed wacker) then make sure you give it an oil change.
I fixed my stihl curved head for bushing seizure / overheat. I removed the plastic head, the orange guard. and the screw holding the bushing assembly (oiling hole). I then pulled the seized assembly from the tube. The non threaded hex on the black shaft that drives the head was removed from it’s failed knurling retention by drifting from the assembly such that I had the hex in my hand. I drilled and tapped the hex for a 8-32 set screw. I replaced the hex on the black shaft with adequate (.040”) bushing endplay observed. The (original failure no endplay). Then I took the 1/8” drill sized for tapping through the threaded hole and drilled the shaft .050” deep. I then placed a 8-32 x 3/16” set screw, socket style with a hex allen wrench into the threads with red loctite applied to the threaded hole. To replace the bushing assembly I drilled the trimmer tube with a 1/4’ drill bit for access such that a cabinet straight blade screwdriver could deflect the drive cable to the middle of the tube. This hole was placed 1/4” above where the assembly would be located when assembled. To cover this hole I placed 3M electrical tape around tube followed by a #12 screw hose clamp over the tape. A temporary fix for this type of failure would be to drift the bare shaft up into the hex below the bushing for .040” endplay, and use a blade style head that never needs to be tapped for line extension. Tapping the head on the ground drives the hex upward while the flex drive is pushing the shaft down during operation, resulting in seizure of endplay. This occurs easily under continued loads of tall grass or weeds.
Stihl USA rarely sells this service shaft/bushings assembly.
Also check there is no weeds or extra trimmer line wrapped around the shaft causing friction and heat to build up. Check (WITH MOTOR OFF) if the spool is easy to turn and free of debris buildup. Remove spool to check for excess movement from the shaft as said previously. This may be as simple as replacing the shaft bushing or bearing depending on which model you have.
I was using my Craftman #316.711023 weed whacker to do normal trimming. After using it for about 15 minutes...not over working it....not just continually gunning it at full speed.....just normal use.....when all of a sudden the cutting head assembly started wobbling & smoking. I then noticed the cutting head was melted and that was why it started wobbling. I think the bearings right above the cutting head got too hot. Nowhere in my owners manual does it say anything about cleaning or lubing the bearings.
I think this model has a defect because this is the 2nd cutting head assembly I am going to have to replace. Last year it did the same thing.....I just didn't realize WHY it happened. This was only the second time I've used the trimmer this season. We have a 3/4 acre yard so I shouldn't be over working it. THERE IS DEFINITELY A DEFECT!!!