ORIGINAL: Dr1Driver
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.
You mean RC59, right? The RC9 is a cold plug designed for high nitro applications.
For 4 stroke engines, you may need a special plug. I'll leave the answering of that question to the 4 stroke gurus.
I use OS F for 4 strokes and OS #8 for most of my 2 strokes. I actually use lots of RC9s too, great plugs, but in boats where I run 50% nitro.