Thanks Dick!,
I just hoped that someone would help me out! David, thanks, but I am aware of how the Nelson plugs seal as I already have some Galbreath heads. The actual angle required just happens to be that of a regular twist drill if I am not mistaken. Henry recomends that the angle of the seat should be a few degrees greater than that of the plug. That way one gets a good line seal.
Mind you using burnt out glow heads is not a s simple as you would think. The Nelson plug needs to be very much recessed in a standard Cox glowhead. Only by doing this can the bottom of the plug be in the same relative position as in the Galbreath head. To be honest, it really is not worth trying to do this if you are an occasional Cox user. I just like the challenge, but if you are not as stupid as myself, just go out and buy the Galbreath heads, they are made from scratch and are very reasonably priced for the work involved!