Fox RC long glow plugs
Russian,
No, you certainly will not ruin your engine using fox plugs because they are slightly on the cold heat range. If you're using 15% fuel the fox plugs are extra insurance against preignition if you get a lean run. If you're using 5 or 10% fuel you probably won't get the max rpm that you'd get if you used some of the hotter plugs mentioned above. I still use some of these fox plugs and as long as the needles are set correctly I never have deadsticks using 10% fuel.
I'm refering to the fox rc long plugs as I've never used the miracle plug. I cut the idle bar off the fox plugs, but if you do that clean all the cuttings out of plug.
Happy Flying