Glossary
Boss — definition
A local raised feature in the section that receives a screw or a thread.
Bosses are the most common source of shrinkage porosity in a cast housing. Where the boss meets the wall the section doubles, and that is where the metal solidifies last.
The remedy is to core the boss from the back so the wall thickness stays even, and to tie it to the wall with a rib rather than blending it in.
If the boss is to be threaded, the hole must be cored in the casting and given the right machining allowance. A hole cast too tight makes for hard machining; one cast too large leaves the thread without enough material to bite into.