This article goes through the decision in order. It assumes you have already found a motor that feels or sounds different from the others. If you are not there yet, the checks after a crash (spin by hand, wiggle the bell, fingernail test without props) are covered in a separate article.
First, find out what is actually worn
GetFPV's maintenance guide gives the comparisons that matter, and all of them work best against a new or spare motor of the same model:
- Friction by hand. Turn the bell slowly. A motor that feels rougher than a new one needs oil, cleaning or new bearings.
- Side play. Try to rock the bell sideways. There should be very little. If there is clearly more than on a new motor, plan for new bearings or a new motor.
- Vibration under power. Without props, arm and spin the motors, then touch the base of each one with a fingernail. Strong vibration on one motor may mean a bent shaft.
- Straight shaft. Once the motor is apart, look at the shaft. A bent shaft has to be replaced. New bearings will not fix it.
GetFPV also recommends a continuity test with a multimeter from each motor wire to the frame. There should be no continuity. If there is, either the motor is faulty or a motor screw is too long and touches the windings. This has nothing to do with bearings, but you do it with the motor already in your hands.
Ball bearings or bushings
Before reaching for oil, check what kind of motor you have. Oscar Liang points out that the tiny motors used in whoops, such as the 0702 and 0703 sizes, typically use brass (sintered) bushings rather than ball bearings. A bushing creates much more friction, and these motors spin very fast, so a drop of oil makes a noticeable difference: smoother running, less vibration reaching the gyro, less noise and longer flight time. Oscar notes the gain is bigger on cheaper motors.
Larger motors use ball bearings, and that is where opinions split.
To oil or not to oil
Oscar Liang does not oil motors with ball bearings at all. His reasoning: the bearings are sealed with grease inside, so oil does not help much, and it attracts dust and dirt.
GetFPV takes the opposite view. Its guide says to apply high-speed bearing oil to both sides of the bearing, turn the motor slowly to spread it, and re-oil every one to two months.
Flite Test's bearing replacement tutorial notes that some bearings do not require oiling, but its author strongly recommends it anyway.
Dirt is the price of oil in either case. Even for whoop motors, where Oscar recommends oiling, he notes that oiled motors get dirty easily, especially in dusty places. If you fly over sand or dry grass, oil will collect it. A reasonable way to combine the advice: do not oil a ball-bearing motor that feels as smooth as a new one. If it has become rough and you want to try cleaning before replacing, clean first and oil after, as GetFPV suggests, using an oil sold for high-speed bearings rather than whatever is on the shelf.
Oiling a whoop motor
Oscar's procedure for bushing motors:
- Hold the motor so you are looking at it from the side, and put a tiny drop on the shaft inside the bell, just above the stator.
- Turn the bell slowly by hand so the oil spreads around the shaft.
- Turn the motor over and put a tiny drop on the shaft at the bottom. Skip this step if it is a ball bearing.
- Put a paper towel under the drone, arm it and let the motors spin slowly for a few seconds until no more oil comes out.
He suggests making the first flight afterwards outdoors, in case oil splashes.
Cleaning
For bearings that are full of dirt, rusty or rough, GetFPV suggests an ultrasonic cleaner or, as an alternative, spraying each bearing with WD-40, and then oiling them with high-speed bearing oil. For dirt on the motor itself, it suggests a bristle brush or rinsing with water, then drying the motor carefully and keeping water away from the bearings as much as you can.
Cleaning is worth a try when the problem is dirt. It removes dirt, not wear, so a bearing with clearly more side play than a new one is a candidate for replacement rather than another wash.
Replacing the bearings
What you need to know first
Bearings in FPV motors are described by outer diameter, inner diameter and height. iFlight, for example, sells NSK 9x4x4 replacement bearings (9 mm outer, 4 mm inner, 4 mm high) for its XING 2206, 2207, 2208 and 2306 motors. Check your motor's manual or the manufacturer's spare parts before ordering. A wrong size will not fit at all, or will fit loosely.
Oscar Liang's notes on choosing bearings:
- What sets a bearing's size class is the difference between the outer and inner diameter. A wider ring holds larger balls, which handle crashes better. Smaller balls run more smoothly at high speed.
- The inner diameter sets the shaft size. He calls 9 x 4 mm a good balance between durability and smoothness.
- Ceramic bearings are smoother but more prone to breaking.
- NSK, NMB and EZO are the popular brands. EZO is often hyped as the best, but Oscar says its advantage is hard to measure, and that counterfeit bearings are a real risk.
Taking the motor apart
The bell is held on by a clip or a screw at the bottom of the shaft. GetFPV describes removing it and separating the bell from the stator, and warns that this can take some force.
Clips are small, so work over a tray or a clean surface. Flite Test's author recommends new clips: according to him, reusing the old one is possible only if you remove it without damaging it, which is almost impossible.
With a screw, use a driver that fits the head snugly. A stripped screw head makes getting the bell off much harder.
Out with the old, in with the new
GetFPV's method is to press the bearings out of the stator with a press or a vice, and press the new ones in the same way. Two rules apply to pressing in:
- Press on the outer ring. A reader comment under the Flite Test tutorial warns never to pound on the inner ring and suggests a socket slightly smaller than the bearing's outer diameter as a pusher. Force on the inner ring goes through the balls of a bearing that has not turned once yet.
- Do not hammer. The Flite Test tutorial itself warns against knocking new bearings in too hard. If a bearing does not want to go in, its author suggests putting the bearings in a fridge for a few hours so that the metal shrinks slightly.
After reassembly, turn the bell by hand and compare it with your spare motor. It should feel the same.
When to give up and buy a new motor
New bearings make sense when the rest of the motor is healthy. Replace the whole motor when:
- the shaft is bent,
- the bell is dented or the magnet ring has come loose (GetFPV's first visual check),
- you cannot find bearings in the right size,
- the windings look burnt.
Cheap motors are a judgment call. If a pair of bearings costs a noticeable part of the price of a new motor and the job takes an evening, a new motor is often the more sensible choice. The old one can still give you a spare bell or a spare stator.