Ever been at the flying site with your gang and up comes one plane, OS .25 LA-S engine with the right front motor mount hole busted out? Not unusual on trainers. It ran fine before, but this day is just would not start. Not even a pop. Fuel tank definitely full, line clear, filter clear, new glow plug, primed, tried three different glow drivers, has some compression but no popping, NONE. So, Jim removed the engine, gave it to me as I said I'll get it to run. This morning I started the disassembly, assuming some one had got things backward inside. It happens all to often on ebay specials. Right? Well, off comes the back plate and a chunk of the engine falls into my hand.
A hair line crack, not visible when mounted on the plane, allowed all crankcase pressure/vacuum to be gone with each flip, thus no fuel was flowing into the combustion chamber, thus no pop and no starting. Would not even pop with prime. That simple once the engine was out of the plane.
A new crankcase is on the way.