On some of the Evolution .61s on the Alpha 60, we've used OS F plugs to help them run better, depends on the engine a bit though.

I use a K&B 7300 (aka HP) plug in my Evolution that I bought from an ex-student after he wiped out his Alpha 40 and didn't want to fly another small plane. The K&B plug is the same length as the others, a little longer than the OS plugs.

I don't know why OS plugs are slightly shorter than everyone else's plug, but they are. On the other hand, lots of guys at my field do run their Evolutions on OS #8 or A3 plugs and have no problems.

YMMV and how your engine runs on a given plug will depend on your fuel (esp nitro %), your prop, air temp, and the condition of the engine and muffler.

(I run 15% nitro and took the baffle out of the muffler, run an 11x5 APC prop, and make a lot of noise and RPM so the colder K&B 7300 plug makes sense, a hot plug would cause my setup to get into pre-detination, which sounds kind of like an egg frying while the engine runs)