Hi!
You choose the plug after what compression you have and what fuel (Nitro) you intend to use!
The more nitro... the cooler plug!
The more compression...the cooler plug!
Less compression...the warmer plug you use
Less nitro ...the warmer plug you use!

Simple?!
Well it is when you got the hang of it!
But remember that if you intend to use a pipe...then you automatically raise the compression inside the engine (By the function of the pipe) and have to use less nitro...or a cooler plug! Or both ...Your choice!


Remember! All European engines (MVVS, Rossi, Nova Rossi, Webra, OPS) are made to run well on 0-5 %nitro (some can handle 10-15%).
But all other engines can handle 0-15% (some even 20% nitro).
If you want to run these European engines on 10-15%nitro...you better add a 0,1mm aluminum head gasket (shim).
Or if you want to run the others (ASP, OS, Magnum,Tower, Enya) on 0-5% nitro... remove a head gasket!

A simple rule to follow!
All two stroke engines run well on OS 8 or Enya 3 plugs!
These plugs are not the best plugs to use if you want out most power from a Q-500 (424) .40 standard engine, but for a sport flier they are the best! For racing, a cooler plug like the Nova-Rossi 4 or 4A is better...or an even colder plug like the Nova-Rossi or Rossi 5, 6 or 7.
All Four strokes run best on the OS F plug!

Finally...Remember that choosing the right glow-plug and compression (head shim) isn't the only thing that govern how your engines is going to perform, the prop size is also a vital component in how the engines is going to perform.