http://209.15.36.61/rdrc/gloplugs.html As you see all the manufacturers make different heat ranges. If, when you take the glow starter off, the engine slows down, the motor is to rich or the plug is to cold. The plug length is determined by how long the manufacturer makes the thread, be it long or short. Looking down the plug hole, it's pretty obvious whether it's long or short. Plug length has nothing to do with heat range, although it can run hot or cold if you use the wrong plug length in it.