The specs of my bike lists the headset size as "NECO 1/18” Threadless". However, I am skeptical on the sizing because 1/18" is 1.41mm . How do I determine my size?

I was adjusting my handlebars, then lifted my bike and the frame/ fork separated. My bearings now make a bit of noise now- like there's sand when I turn my handlebars. Do I apply more grease to the bearings? Get a new headset? I'm afraid I don't know what I'm doing...