Lister D water drain tap bung removal

I need to flush the water side of the engine, the thread on the tap is stripped and wont tighten, and the thread in the bung is pretty much gone as well. So I was wondering how easy is it to remove the bung plug, with out risking any more trouble. Then I can re drill the bung and fit a 3/8" tap. also without a tap in it, would there be enough hole to properly flush the rust out of the engine. any advice please.
John

The bungs are usually rusted in and are extremely tight! I have removed them by cutting them off flush to the block and then cutting through the thread with a hacksaw blade and carefully removing the thread with a hammer and punch. You can get new bungs on ebay