For a 3-inch build, motor screw choice is mostly about matching the motor base, the arm thickness, and the mount style without letting the screws touch the motor windings. The first thing I check is the motor’s mounting pattern in the spec sheet. Most small motors use M2 hardware, while some larger 3-inch setups use M3, but you should not assume based on prop size alone. A 16xx motor, for example, often uses M3 screws, while many smaller 11xx to 14xx motors use M2. If the vendor listing does not clearly state the thread size, look at the mounting holes on the motor or the documentation from the manufacturer.
Length matters just as much as thread size. The screw needs to go through the arm or motor mount and engage enough threads in the motor base to hold firmly, but it should not bottom out inside the motor. That is the mistake that causes damage. As a rule, I measure the arm thickness first, then add only the amount needed for the motor base plate, usually aiming for enough bite to get secure purchase without excess length. On many 3-inch frames, screw lengths around 4 mm to 8 mm are common, but the right length depends on the frame and whether you are using separate aluminum plates, carbon arms, or top-mounted brackets. If you are mounting through a thick arm, you may need a longer screw, but always check the motor depth before tightening.
Another useful tip is to test fit one screw before installing all four. Insert it by hand and make sure it turns smoothly and does not suddenly stop too early. If you feel resistance before the screw head seats, stop and recheck the length. Never force a screw into a motor. A screw that is too long can scrape the stator windings inside the motor and ruin it immediately, even if the outside looks fine.
I also prefer quality steel screws with the correct driver size, because cheap hardware can round out easily and make later maintenance annoying. If your frame uses countersunk hardware elsewhere, note that motor screws are usually button-head, not countersunk, since you want a strong head and predictable clamping. Using a dab of medium threadlocker can help on metal-to-metal mounts, but do not use it on plastic parts unless the manufacturer says it is safe.
If you are unsure, buy a small assortment of M2 and M3 screws in a few short lengths. For a 3-inch build, that saves time and lets you match the exact stack-up instead of guessing. When in doubt, shorter is safer, as long as the screw still fully holds the motor.
