Hi Doc
Don’t waste your time trying to procure a tap for the Nelson plug as reworking Cox plugs to take the Nelson is not a simple process (the subject was covered extensively on the r/c universe forum) and you will need additional tools to drill and machine the plug seat which is critical.
Save yourself a fortune and buy a Galbreath head for the Cox 049 and a plug direct from Doug and you will be in front right from the start. If you don’t believe me price up the tap and you will find you can buy several heads and spare plug for the same amount of money.
Oh and Doug’s site is in English
Ployd in OZ