Short plugs are used only in smaller engines, usually under .15. Idle bar plugs have a bar across the face of the plug, partially obscuring the element. This supposedly prevents raw fuel from hitting the plug and drowning the element. In my opinion, idle bar plugs are only for people who can't properly tune their engines.

In 2 stroke engines from .15 to 1.20, the same plug can be used. I've used McCoy RC 9 or 59 plugs in everything I run for over 20 years. I'm very happy with them, and I don't think there's a better plug around. It's a tough plug, medium heat range, and works well with anything from 10% to 25% nitro.

For 4 stroke engines, you may need a special plug. I'll leave the answering of that question to the 4 stroke gurus.

Dr.1