I understand that Henry Nelson sold the tap for his plugs. Now that he has retired (?) anyone know where you can buy one without spending a fortune? I thought I might try my hand at making some Galbreath type heads for my Coxs. Being a bit of a hoarder, I have lots of burnt out Cox heads!

Regards,

Andrew.

  Would that be a 1/4"-32 tap??  That is the size for all others that I have..

Here you go!

http://www.tapsndies.com/catalog/item/6890755/6857040.htm

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!

         Andrew, one of the problems I was faced with when I tried this was just as you mentioned. In order to get the proper depth of plug, you sacrifice the integrity of the notches for the head wrench. This doesn't mean that you end up machining them off. I was able to do it, but after tightening the Cox head using the factory wrenches, it more or less ripped the notches off of the plug. There was little material left surrounding the top of the plug. It did work though. If you take note of the Galbreath head for instance it is slightly wider than the stock head and the notches are machined with radiused corners. Ken

If you have a machine that's capable of threading, it may be easier to just make heads from bar stock.  I made my first head for a standard glo plug by converting a Cox head.  The second I made out of bar stock, and I was happy I did.

Also: you may find that just using a twist drill gives you the correct overall taper, but not a good enough finish -- I suspect that what will happen is that sometimes you'll get a nice finish, and other times you won't.  To do this reliably in production you'd probably want a reamer, and a custom one at that.  But if you can sharpen your own drill bits, you can make sure to keep 'the special one' nice and sharp and clean just for this one task.

It's 40TPI, it appears to be a normal North American thread form.  But I can't remember the OD.