It sounds like you hydro locked the engine even though you thought you had pumped all the oil out of the engine the crank case can hold alot of oil and when you turn it back over and put the plug in it, it was just the right amount of pressure to push the rest of the oil from the crank case and I hate to say it but that 28 you're running had a really soft piston and a really hard wrist pin and more than likely you ovaled the slot in the piston where the wrist rides. or it could have happened on the other end at the crank and the piston rod.