To glue or not to glue?

While bleeding the fuel system after carelessly running out of diesel (another story!) I managed to break off the nut-like piece off the bleed valve on the fuel injector pump. I noticed that it wasn't a rough shear as the surface of both the piece that came off and the body of the pump are smooth machined surfaces. The question I have is how to get it stuck back on? It guess it must have been chemically bonded originally so I was hoping to just glue it back on but don't know if this is wise nor what product or method to use. Anybody out there with any advice?

Thanks,

JonnyB