Save your money a standard K&B 4520 plug will work every bit as good and last just as long as a OS or Enya at a fraction of the cost. Another advantage of the K&B is the Idle bar - when the fuel charge hits the plug after an extended idle, (autorotation etc.) the bar keeps the element at a higher temp to help prevent flame-outs.
When I flew competition - OS and Enya gave me plugs my box was full of them, after an extended test for years in competition and aerobatics I always went back to the 4520.

If you don't believe me - ask Curtis and his dad, I turned them on to the plug years ago and they still use them today.

Mike Mas