May sound like a dumb question, but will a new rotor in an old stator be a like new motor? I’m ignoring bearing wear and assuming it’s shimmed correctly, will it spin near new?

Depends on the condition of the motor. If the old rotor has faded but the stator is still in good condition, yes. Otherwise, no.

The stator can only go bad by burned wires. As long that is not the case it will work as new.

Id do the bearings too.

Negative, it can also be damaged by foreign objects eg screws getting jammed in the motor. Or a crash bending the can etc.

True but then you must know something has happened with that motor. But from the point of view with a normal use w/o hard crashes there is no decrease of performance with the stator

Chances are if you cooked the rotor causing it to lose is magnetic properties, then you likely cooked the stator as well. If there are black burn marks anywhere on the coil, then it's junk, you can also smell it and any signs of "burnt electrical" smell then it's garbage. I've learned to replace my motors every 6 months to a year tops, if it's in decent working condition, I'll bundle everything together into a RTR deal and buy all new equipment in order to be competitive.

Basically what Billdelong stated. Check for discoloration of the windings. How old is the motor?

A year, maybe two but i average going to the track 3 times a month so not high use. I want to switch to an rpm motor but being kinda cheap about it. I’d rather get a donor motor, and rpm based rotor. I’m deciding between this route or just timing and gearing to the moon to take away some of the torque.

I’m not trying to destroy it, I still hook it up to the motor analyzer and graph it, but more in the amp spike rather than at the beginning.

6 months? That's a bit excessive. Brushless motors should last several years if treated properly.

You should be able to get more than a year out of a motor. Copper doesn't really start to change color until around 200*F. Rotors are not really designed to handle 180*f. Needing to change your motor out after 6 months seams a little excessive, or you are running your motor to it's temp limit. At most, you might need to swap out the rotor and bearings, but I'm sure your stator will be fine.

As long as you're not letting our temp reach above 160*f, your stator should be fine. I would just slap a different rotor in it. Is this motor the Fantom Helix V1?

It's not the copper discolouring, it's the enamel coating.

The enamel coating can handle more heat than the copper. Doesn't it handle over 250*F? That's why I didn't bring up the enamel. There would be many more issues if the motor reached over 250*F.

Depends on the enamel type used. Hobbywing recently changed to a high temperature enamel that can do 220C, but there are types that fail around 105C (220F). I'm not sure what the typical rating is for most motors on the market right now.

If the stator wire is going black, that's the enamel discolouring, not the copper. Copper can handle way higher temperatures. The rating is generally what the insulation can handle.

As for the motor reaching 250F, don't forget that the motor isn't at a uniform temperature. The windings can be hotter than the can or the temperature sensor on the sensor board.