These are cut and paste quotes from the Tower web site.
O.S. 8 Glow Plug Standard Long Medium Hot
O.S. A3 Glow Plug Hot
ENYA #3 is a Hot Heat (for 0%-5% Nitromethane Fuels) Long Glow Plug for
Any Engine that uses a Long Plug (Not specifically for Enya engines).
ENYA #4 is a medium heat (for 5%-20% Nitromethane fuels) long glow plug for
any engine that uses a long plug (Not necessarily for Enya engines).
TOWER POWER GLOW PLUG - Medium Length,
Medium Heat Range Plug WITHOUT IDLER BAR.
Works on All 2-Stroke Engines, even .15 and smaller because it does
not have the idle bar that would normally hit the top of the piston