• This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn more.

Unknown Brushless Motor Help

I received 3 different brushless motors from a guy at my flying field today. They have no markings on them. How would I figure out the prop size to use.
One motor is 35mm in diameter by 42mm tall (no shaft).
The second is 37mm in diameter by 37mm tall.
The last is 38mm in diameter by 14mm tall (motor bell size).
Any help is much appreciated.



Active member
Try searching the sizes in mm with diameter, then no space, then height. ex- 5010 for a 50mm diameter, 10mm height.
Try using bell sizes or stator sizes because some companies go by bell, others by stator, and others just are arbitrary numbers.
I don't recognize any of these.


Legendary member
I agree with @Dante, most manufacturers measure the size of the stator, some measure the size of the bell. Not sure which you measured. It would be good to search for both measurements.

You will likely need to do some bench testing. Start with a smaller prop and work larger unil the motor starts getting warm. Your looking for something that would feel good holding in your hand on a cold day. If you need to let go right now, that’s too hot, back off the prop size.
Last edited:


Elite member
Another good rule of thumb is that taller motors tend to be lower KV and run longer props, so the one on the right is most likely sub 1500kv and designed for 9+ inch props.