The easiest way to remove them and also pipe plugs is to thermal shock them. to do this you heat the nipple or pipe plug being removed with a torch. do not heat the cast housing. The nipple or pipe plug does not have to be red hot but needs to be hotter than a propane torch will get it, so it works best with an oxy-acetylene torch. Then douche the nipple or pipe plug with cold water wait about 5 seconds and remove with suitable tool. the nipples normally require a pipe wrench. Good luck.
