The data sheet I have for a Dayton motor states that it can handle a 10-15% over or under voltage. So if you do the math, a motor rated for 110 volts is OK at 120 volts.
However, there is a fly in the ointment. How are you measuring the voltage? If I remember correctly a standard type VOM measures peak-to-peak voltage, and a motor runs on RMS (root-mean-square), which is less than peak-to-peak. So you may not be seeing a true 120 volts if your measuring the line with a VOM. You would have to put an oscilloscope on the line to find the RMS voltage, although they may make a meter that reads RMS, I haven't thought about this stuff for 20 years, and I'm sure they make test equipment that can do anything these days. Maybe an "app" on a cellphone, LOL.
The electrical engineers will let us know if my memory is "bad" about what I've just said. I myself wouldn't worry about it. A little over voltage is better than low voltage, anyway.