I don't believe it is necessary to break them in on Glow, the Enya .25 Diesels Martin and I just got are ABC and didn't come with Glow Heads. The Irvine .40 Diesels are ABC and don't come with Glow heads. Dubb Jett says that the ABC style engines adjust themselves to whatever temperature they're being operated at. The perfect piston to liner seal of my Fox .74 ABC seems to have lasted longer as a Diesel than it would have as a glow engine, about 12 years now. Based on my years of fooling with and working on engines since I was 10 years old, now 62, I believe breaking them in the way you're going to use them is the way to go. I haven't messed one up yet and I currently have 120 engines.