As an aside, hydrolock also plagues “round” engines on full-scale airplanes. You might have seen documentaries or newsreel films of ground crews pulling propellers through on B-17s and B-24s during World War Two. This was to ensure against hydrolock. Then, when the crew got on board and started the engine, one of the pistons wouldn’t hit a “brick wall” and damage a connecting rod, or something. Another term is simply “oil lock.” On radial engines the oil will tend to drain into the lower cylinders. If enough oil is in there during the compression stroke, the piston will hit a “brick wall.”

If you remember flying on DC-3s, DC-4s, DC-6s DC-7s or Lockheed Constellations (these were “top of the line” airliners at one time), then you might have noticed that the engines never started “right away.” This was because during the starting procedure the crew would count eight blades of rotation before turning on the magneto (“ignition”) switch. Once again, this was to prevent hydrolock. It took only six blades of prop rotation to clear all cylinders on the R-2800 engine that powered the DC-6, but it took eight blades of prop rotation to do the same for the R-3350 that powered the DC-7 - - which had a four-bladed prop. So, just for standardization, eight blades were counted during the starting sequence on ALL of the birds before turning on the magneto switch to provide ignition.

The same hydrolock problem can rear its ugly head on 4-stroke model engines. So, it’s best to rotate the prop through a couple of cycles prior to heating up the glow plug. This would be especially true on 4-strokes that are mounted inverted. You sure don’t want to hit a “brick wall” with the piston, right?

F-89 Driver