i am looking to buy the tool i saw once that is used to (un)mount really short threaded nipples where a regular wrench is too thick to reach. as opposed to reaching it on the outside the nipple, like a normal wrench would do, this thing gets pushed inside the nipple, provided that the nipple has little wings (for the lack of a better word) inside onto which this tool catches on (regular-sized nipples do not have these wings). also, this tool usually comes with several sizes in succession so that you push it in until the right size catches on. then you use a regular wrench on the part of the tool that remains sticking out to twist.
i am sure you all have seen this but i just do not know what it is called. if my description is not enough, i can maybe draw a picture.