ORIGINAL: ffkiwi
There are actually three Nelson plugs Graham-the std-the original, the HD (heavy duty) designed for high compression and a bit of nitro-mainly used by combat and F1C fliers, and the newer 'hot' plug (which is currently available from Larry Driskill)
There's actually four. There's the two part "Glo Bee" style plug, with the flat spiral element. Mainly used for F3D (Pylon Racing). Same thread as the other Nelson plugs. Haven't tried one in my F2D engines as yet.
[link=http://www.pspec.com/details.asp?ProdID=37&category=7]http://www.pspec.com/details.asp?ProdID=37&category=7[/link]
There are also various Chinese and European copies - the Chinese ones are made by CS, I believe there are two heat range options. You get these from Billy Hughes (the F2A Speed flier). Adriano Molteni sells a European one. The Russians and Ukrainians were also re-elementing burned out plugs for a while. probably still do. I have a few of the remanufactured plugs (you had to trade in a "core" when you bought one).
Anyone contact Al Kelly (of Merlin Glowplugs)? He supposedly re-elements burnt out Cox glow heads (and other heads as well). If I ever get my Tee Dee .09 running I might ask him to do the heads when they burn out. Or send them to Nelson or someone to get converted to Nelson or Turbo plugs.
Iskandar