In most engines long/short it really doesnt matter alot but its best not to run a LONG plug in an OS engine because they are designed for a short plug(probably one of the ONLY engines that uses them ) As far as standard(normal) plugs,standard=Long shaft "Non-Turbo" plug,For me Odonnell 99 plug in .21`s as well as in my OS.18 TM in a Revo work very well(The ONLY OS engine designed for a standard plug )But go with what works best for your situation