For a 7-inch build, the safest approach is to match the motor screw length to the motor’s mounting plate and the thickness of your arm, then verify that the screws do not bottom out inside the motor. That last part matters more than almost anything else. If a screw is too long, it can touch the windings or stator inside the motor and cause damage that may not show up until the motor starts overheating, cogging, or failing in flight. A quick dry fit before final assembly saves a lot of trouble.
Most FPV motors use M3 screws, but you should not assume that without checking the motor spec sheet. Some smaller motors use M2 or M2.5, and forcing the wrong thread can strip the base immediately. If the manufacturer lists a maximum screw depth, follow it exactly. If not, measure the thickness of the motor mounting flange and the arm together, then choose a screw that gives full grip without poking through too far. On many 7-inch frames, 6 mm to 8 mm screws are common, but the correct length depends on the frame arm thickness and whether you are using countersunk or button-head screws.
Material matters too. Steel is usually the best all-around choice because it is strong, inexpensive, and handles vibration well. Titanium is lighter and corrosion resistant, but it is often unnecessary for motor screws and can be more expensive than it is worth. Aluminum is generally a poor choice for motor mounting because it is too soft and can shear or loosen under vibration and crash loads. For a 7-inch quad, durability is more useful than saving a gram or two here.
I also recommend using quality hex-drive screws from a reputable hardware source rather than random mixed hardware. Cheap screws can have sloppy heads, poor threads, or inconsistent length, which makes installation frustrating and increases the chance of stripping. If your motors sit on thick arms or carbon with a recess, make sure the screw head style seats properly. A screw that barely grabs threads is worse than one that is slightly heavier but fully engaged.
If the motor comes with a lot of thread engagement already from the arm thickness, do not force an even longer screw just because it feels more secure. You want enough engagement for strength, but not so much that the screw reaches the motor internals. A good habit is to install one screw at a time by hand, remove it, and check for any mark or interference. If there is any doubt, use a shorter screw and add a thin washer only if the motor still sits flat and aligned.
For thread locking, a small amount of medium-strength threadlocker on metal-to-metal threads is sensible, but avoid getting it inside the motor. Do not use threadlocker if the screws go into aluminum without confirming compatibility, since it can sometimes make future removal messy. Clean screws and dry threads help the parts stay put better than overdoing the compound.
If you are unsure, the simplest answer is: use the screw type and length recommended by the motor maker, choose steel, and confirm that the screws do not protrude too far into the motor. That is the combination most experienced builders rely on because it is strong, predictable, and easy to service later.
