E MOTOR & FILAMENT SENSOR COOLING FOR BONDTECH UPGRADE PRUSA MK3S+

 On all four of my Prusas, the sensor has failed after printing at this temperature for several days (the sensor falsely triggers even with filament inserted). Therefore, I assume this is due to the high temperature the sensor reaches (inside the cover it might be around 100°C), but I cannot be completely sure.
Cooling the motor is not strictly necessary, but it extends the motor’s lifespan and preserves the plastic parts in contact with it (ABS in my case). It also helps reduce the heat transferred through the shaft to the internal plastic gear.

I use this with MarcoZ76’s BMG upgrade (I’m not sure if it’s compatible with the original Bondtech). I print it in ABS on a smooth PEI bed, with glue stick, no brim, chamber temperature at 55-60C and part cooling set to 30% (if using BMG — if you’re using the stock Prusa extruder, increase it to 40–50%).
Supports are required, and after removing them, you’ll need to cut the connecting bridge between the two “wings.”

You’ll need two M3x14 screws (up to M3x20) to secure the fan, and the same M3x10 screw for the filament sensor cover.

Additionally, I’ve added a front heatsink to the motor so that when printing cooler materials like PETG, I don’t need to turn the fan on — the heatsink alone is enough for cooling (when printing PETG inside the enclosure, the chamber temperature can reach 40–45°C).

I recommend using high-flow fans such as SUNON — do not use NOCTUA fans.