The flapper valve you were refering to is a crankcase breather somewhat like a PCV valve in a car. If they stick, it makes the greatest oil fogging machine you've ever seen. Spent 35 yrs running a small motor shop working on those suckers plus many others. Wouldn't make a good plane motor as it's much to heavy for the power it puts out. Friend of mine and I had a Quadra 200 that weighed about the same but put out 24 hp.