My question is how tight are the motor shaft screws supposed to be? Even if I can find a spec, I have no way to measure.

My motors have a shaft screw holding the bell on, not the C-clip of doom and despair. I took them apart to clean the debris out of them after a few hard hits in the dirt and mud. I was suprised how much dirt was in each motor!

After reassembly the motors were very tight and would barely spin by hand. I loosened the screws to what I thought was how they originally would spin by hand. Did a short, gentle test flight and they were VERY noisy and hot. Double checked to ensure all of the washers and such were in the right place and nothing appeared distorted or out of alignment due to over tightening. Bearings rotate smoothly with no indication of damage or wear. Being a mechanic I have a pretty good feel for how tight things are when I disassemble.

If I go too much looser I will be concerned about the screws falling out. If I do nothing they aren't going to last too long. My thoughts lean toward getting them just tight enough to spin freely with as little axial free play as possible and using a drop of blue thread lock (didn't appear to be any on the screws when I disassembled). 

How do you guys do it when reassembling your motors?

You can't overtighten that screw. As you said, tight just enough and a bit of lock tight. Be careful not to get the lock tight everywhere.

Thanks. I did just like you said and took it for a spin. Seems to be back to normal.

Sounds good.