From over 50 years of having watched film clips from WWII of air crews walking through the props prior to starting the engines on USAAC 4 engine bombers, I had understood they did it due to hydraulic lock. I am talking about TV shows like Air Power back in the 1950's, stuff like that. Seems like every episode they had clips of the radials being turned over piror to start-up for the latest mission that was being documented in the show. I can't remember whether it was always the same clip spliced into each weeks episode, or even if they were different planes (like were the B-17 engines more susceptable to hydraulic lock than B-29 engines). Like I said, somebody who had been in the AAC during the war told me it had to be done. First time I flew in a radial engine equipped plane, probably a North Central Convair 340 wtih R2800s, they just fired up the engines and off we went. Progress, or television show film editors splicing in bits of good sharp film to make sure they had enough to fill a 30 minute episode?