I think that you will find that they are a 'taper-thread'. The best method is to put a short 1/2in drive extension on it THE WRONG WAY ROUND---i.e.the 'femal' of the extension on to the drain-plug, and then either a socket on the 'male' end or a long adjustable. As previously suggested---fit a new drain plug. I have found that the easiest way to fill the gear-box (if you have to) is to get a length of clear (so that you see the oil drain into the gearbox) plastic piping that will fit INTO the drain/fill orifice (posh word for--hole) and is long enough that it will extend up pass the engine in the engine bay, put a small funnel into the plastic hose and fill the gearbox that way. Of course, if you already have a oil-syringe for filling rear-axles and gearboxes, use that!