Expert: Marc Ebelini - 10/20/2005

Question
Yours is one of very few AllExperts profiles which may be compatible with the nature of my question, so I hope you can help me out.
I was an aircraft hydraulics mechanic forty years ago in the Marines but worked only on turbo props and jets.  Consequently I never knew why it was necessary for the big reciprocating engine propellers to be turned by hand back in the WWII era. It's something you often see in documentaries from that period, especially on the heavy bombers.  Was this to reset the magnetos or somehow line up the shafts and cams?  Thanks in advance.

Answer
This is an easy one. The propellers had to be pulled through at least two revolutions to make sure oil had not accumulated in the lower cylinders. An attempt to start the engine without doing this could result in a hydraulic lock.
The momentum of those heavy propellers would damage the cylinder base flanges if one of the upper cylinders fired before the oil filled cylinder had an opportunity to purge itself during an exhaust stoke. If the propeller came to a halt during the rotation because one of the cylinders with oil was coming up on a compression stroke, the Spark plugs had to be removed to drain it. This was also the practice for inverted engines.
May I contribute another bit of information? Did you ever see flames coming out of the exhaust when one of those engines started? That was the oil that had settled in one of the lower cylinders. It was important to keep the engine running and give it a surge of RPM's to blow it out. I hope you are satisfied with my explanations.