Here are a couple numbers that may help you decide how hot is too hot.
A NEMA insulation temperature tolerance class A (the lowest rated class) motor is rated to operate at up to about 220 deg F. I don't know the rating of the Gast motors. Import motors are probably not as good.
Anything over 120 deg F can get painful quickly if you're sensitive. 130 and up can quickly result in burns if the heat transfer rate is good, like with damp hands.
So yes, if you can keep your hand on it it's ok is correct and leaves a good margin for error.