so i binned my 5inch pretty hard into a concrete barrier at the track last weekend, hit it at maybe 60kph give or take. the arm cracked about halfway through on the front left, not fully snapped but definitely compromised, you can see it flexing when you put any pressure on it. the weird part is all four motors still spin up fine when i plug in and do a manual spin test in betaflight, no grinding, no weird resistance, rpm looks consistent across all four.

my question is basically - do i replace just the arm and leave the motor, or should i just swap the motor out anyway since it took the impact? im using T-Motor F40 Pro IVs on this build and i know they're not cheap but i also dont want to be flying with a motor that looks fine but has bent shaft or something internal thats gonna fail mid flight. anyone dealt with this before

honestly if the shaft isnt visibly bent and the bell spins smooth with no wobble i'd probably leave the motor and just fix the arm. those F40s are pretty tank-like compared to some of the cheaper stuff. what i always do is pull the prop off and spin the bell by hand really slowly while watching it from the side - if theres any wobble at all you'll see it pretty clearly. also check that the motor screws didnt loosen from the impact because that can cause vibration that looks like a motor problem in blackbox but its actually just the mount.

that said if you have a spare F40 just sitting around theres no harm in swapping it for peace of mind. but i wouldnt go buying one just because of this crash unless you actually see something wrong with it.

replace the arm for sure, a cracked arm will eventually let go and a compromised frame is just asking for a worse crash. as for the motor i'd check the shaft with a dial indicator if you have one, otherwise the spin by hand trick works well enough. i've had motors that seemed totally fine after a hard hit and then showed up as noisy in blackbox logs a couple flights later. sometimes the bearings get slightly damaged and it only shows up once they warm up. maybe do a short hover test after you fix the arm and check your blackbox for gyro noise before you go ripping again