The Ryobi and Homelites have a flat plug seat and come with the CJ7Y but actually either plug will work and seal fine. The McColluch comes with a DJ8J tapered plug but you can run the gasket jobs in them if you want to. It doesn't seem to bother the head to swap back and forth.

But whatever or whichever, make sure its the resister version of that size that you use before having the engine in a plane.

I could never tell any difference in the way they ran with either plug but I've always flown them with the type the came with.

Enjoy

Jim