Try JB weld...
Problem is, the cracks are hairline ones, inside the threaded area, extending out to the outer face; getting the oil and contaminants out of the crack prior to doing the JB thang, just doesn't happen, and the only way to really do it is to apply a lot of heat to expand the metal.... problem there is... and I caution not to do this... when you use the solvents that do penetrate and cut the oil, those specific chemicals are ALL deadly when heated to vapor point..., (chloranted solvents like BrakeKlene, Trichlor, and pentachlor types ) they create Phosgene gas, extremely deadly, and even breathing a whisp of the vapor can do major harm to you, irreversable damage to lung, kidney, and liver function, and also long lasting nerve damage... also irresversable..
I mistakingly subjected myself to this happenstance during a cleaning of parts prior to welding some, and within moments knew something was drastically wrong. I still, to this day, 8 years after, have a loss of nerve sensitivity in fingers directly caused from this. I still on occasion, get complete debilitating and painful cramping of my hands, where they clamp up like crab claws, uncontrollably after doing intesive wrenching. The first occurance of this was during the initial exposure, so i know for sure this was the cause. I also underwent some severe resperitory issues, and other symptoms for the year following the initial exposure, so I caution people about this.
The amount of vapor I inhaled to cause all that, was about half of what you would see by striking a single match, just one breath of it before I knew it was "not good"..