✔ 最佳答案
You can try getting a reverse twist drill bit. Just a little under sized. This works if broke off flush. There is also a stud remover tool if it is treaded into the head and some projects out. There are at least two variations on this. Irwin tools makes one and the other can be found at sears (and maybe other places).
More then likely the head could use some work any way! Three angle valve grind and other freshing items.