It isn't just plastic backplates. A couple of years ago I was running a Fox 35 in super slow combat. My pitcrew spent 5 minutes cranking and didn't even get started. Problem was a loose backplate. The same plane and motor had been flown several times the day before and it ran just fine. Since the backplate had the fine Fox cork gasket, just tightening the screws again did the trick. It ran fine for another year, before we switched to another brand of combat.