So... I seriously doubt you blew your block/piston/rod, because you'd know it. lol. But I do believe your head gasket is leaking. While you mention this one trip, it's not that quick. I'm talking a tank every few dozen or so cold starts. It's when the engine cools down that...
Each orifice plate was sealed to the top surface of the OWC chamber using a silicone gasket adhesive, which was removed and reapplied each time an orifice plate was installed. To avoid slamming events during testing, the total freeboard of the OWC chamber and orifice is fixed at 20 cm. ...