On my third motor for my RY40021 single string line trimmer. High grass wraps around the 5mm spindle above the head and over time binds the motor and slows it down until it overheats and burns up. Hopefully this will keep the grass out and save my new motor.

It can be installed without disassembly of the whole trimmer housing. Back off counterclockwise the trimmer head. Clean all debris from spindle and area. There are two Philips head screws holding the motor, take them out. The motor will move down some but not far. Add printed shaft saver and use two new 5mm x 12mm long screws. The new part adds 3mm more for the original screws to go through. Don't use too long of a screw that it can reach the armature and destroy the motor. When adding the new screws pull up on the motor shaft against the mounting plate and align to start the new screws. Spin trimmer head back on and check clearance between trimmer head and new part. There should be very little space. If it rubs take part back off and sand top surface to fit.

I just reassembled mine and have not tested it in the yard. After cutting a little I plan to periodically check clearance. I don't want this to be the cause of motor burn up.

I printed this in PLA, 20% infill, 0.2 layer, with support for the recess in the bottom.

Tips: The string eventually wears a slot in the aluminum sleeve in the trimmer head. Put a flat blade screwdriver in the slot and turn the sleeve in the housing for new area for the string to ride against. 

Once the trimmer head is off cut a groove across the end of the spindle shaft. I used a Dremel tool thin abrasive disk. Then you can use a small screwdriver in the hole to hold the shaft while spinning the trimmer head. Pictures shows what I did.

5/31/2023 Update: The trimmer was used about a half hour and taken apart to see the results. Pictured is the amount of grass around the head. No grass was around the spindle shaft. Due to the spinning head with the grass around it, it did wear down some of the plastic of the printed part. If it continues and gets around the shaft, I can always reprint another part and replace easily.

7/3/2024 Update: Time and use finally caught up. Enough grass got around the narrow area around the spindle and wore away the plastic. With more grass came more heat against the plastic and melted it. Had to disassemble the head and clean it. I have a newer version that goes higher around the string head hub. Since last year I bought a roll of PETG with carbon fiber to make a newer model. I am back in business and waiting to see how the revision works.