There are actually 3 lengths of plugs. The left side plug below is a short reach and it's generally only needed in some very small engines. The centre plug is a medium reach and these are only made by OS and Enya although no one has any explanation as to why both these Japanese manufacturers went for that length other than they'll fit all sizes of their engines. The right side is a long plug, obviously ). Ideally, the correct length plug to use is the one that ends up being flush with the combustion chamber. Instruction sheets usually say what length to use but if in doubt it's just a matter of taking off the head and screwing a plug in to see what fits best.

Ah OK, I can't help adding that there's no such thing as a "nitro" engine . They're called "glow" engines. I have maybe a hundred engines but I don't use nitro in the fuel so I can't possibly have any "nitro" engines .