This Dremel Model 370 Moto-Tool is one of my longest-owned tools. It was bought sometime around 1976. After 40 years of use, it began exhibiting erratic behavior due to the motor brushes finally having worn down to where they were not making good contact with the commutator. You can see in the photos how the original brushes did not wear evenly, although the contact surfaces were clean and not pitted at all. I decided that given the Moto-Tool's age, it might be a good idea to disassemble it and do a good cleaning and inspection.
Overall, the Dremel was in very good condition. It has always been well cared for and kept clean and dry. A toothbrush and some isopropyl alcohol did a great job of cleaning all the balsa and plywood dust off the case and internal parts. Even though it wasn't really necessary, I let the magnet assembly soak overnight in a jar of Evapo-Rust. As you can see from the 'after' photo, everything looks almost new.
A light burnishing of the commutators was done with 1200 grit wet/dry sandpaper, followed by a good alcohol cleaning to make sure there were no residual metal or carbon particles.
Reassembly went easily. The new brushes had to be purchased from an aftermarket manufacturer in China because Dremel no longer supplies them for the Model 370. These brushes do not appear to be as high a quality at the originals, so time will tell if they can last four decades.
After breaking in the brushes with a series of slow-to-fast-to-slow cycles, and allowing time to cool down in between, the Moto-Tool runs very smoothly and with good torque. The reconditioning was a total success. I have used the Dremel Moto-Tool few times since, and it works very nicely.
Posted June 17, 2017