Upset Underfill : A depression on the outside of inside surface of an upset caused by insufficient flow of metal to completely fill out the upset to the desired shape. (Bul 5T1)
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}