I don't build engines, but I've owed many.
I'll disagree with my esteemed colleagues. They will all leak. It's just a matter of time.
As the character Red said in Shawshank Redemption: "Geology is the study of pressure and time. That's all it takes really, pressure, and time."
Just substitute "Engine Leaks" for "Geology". Pressure and time. That's all it takes. Poor engine assembly habits just help matters.