O2 sensor on my Vance and Hines popped out while riding. Does anyone know if there's a washer that's supposed to go in between the sensor and the pipe mount, or do I just screw it back in as is?

 

Was it a sensor, or a plug ? The plugs for mine have no washer.

 

o2 sensors for cars normally have a small crush washer kind of like a spark plug. been so long since i did my pipes I cant remember if it had the washer or not. if you torque it tight enough it should not leak. something you should check time to time, cause as you've found out they can loosen up.