If the threads are not too screwed up & can be reworked, a thread file is what you'll need. Unfortunatly it will only work on the male threads, the female half will need to be chucked up in a lathe and chased. Considering machine time will probably hit the hour mark you might ask yourself "Self, why don't you buy a new grinder?".yeah that sounds about right... the groove will likely get messed up, but if you have a tool to remake the groove you'll be golden.
It will be cheaper in the long run.
http://www.amazon.com/KD-Tools-Thread-Restorer-Sizes/dp/B000VXIXUK