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In footage from WWII you often see the ground crew rotating the propeller of a radial engine a number of times before the pilots start the engine.

I know it's not the same as hand-starting (hand-propping).

I read somewhere (but questionable source) that it has to do with the oil sitting down in a radial engine.

As far as I know, the oil surrounds the cylinders on the outside. If the oil is cool and not yet expanded, what will a few turns do to it?

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Radial engines - especially older large-bore designs like you find in WWII era warbirds - can suffer from a phenomenon known as hydraulic lock.

Basically while the engine is off and cooling some oil from the crankcase seeps past the piston rings in the lower cylinders, and collects there. When you try to turn the engine over with this oil in the cylinder the bottom piston contacts the oil (which is an incompressible fluid), and can't complete its travel down into the cylinder.
The stresses in the engine build until something - usually the connecting rod for that bottom piston - fails, resulting in an expensive repair bill and a grounded warbird.

To avoid this aircraft that are going to sit for a long time usually have a spark plug removed from the bottom cylinder and a hose run to an oil jug to collect the seepage.
In addition to this (or in place of it for shorter-term storage) the ground crew usually rotates the propeller through by hand enough times to ensure the bottom cylinder has gone through a compression stroke.

If there is oil in the bottom cylinder the crew will feel the hydraulic lock as extra force required to turn the propeller. They can then stop and remove a spark plug to drain the oil from the cylinder before attempting an engine start.

A similar phenomenon exists with rotary engines as Simon described in his answer, and the engine is pulled through for the same reasons.

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    $\begingroup$ You know what? It wasn't until I read your answer that I realised the OP was asking about radials and not rotaries! Duh. $\endgroup$
    – Simon
    Jun 24, 2016 at 22:08
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    $\begingroup$ I have seen exactly one running rotary engine. It was a totally mind-blowing experience for me - "What do you mean the CYLINDERS are moving? WHAT SORCERY IS THIS?!" $\endgroup$
    – voretaq7
    Jun 25, 2016 at 1:11
  • $\begingroup$ Also notable that a vertically-opposed engine could suffer from hydraulic lock in the lower cylinders as well, but I can't think of any aviation applications with such a configuration because engineers don't hate mechanics that much :) $\endgroup$
    – voretaq7
    Jun 25, 2016 at 1:14
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    $\begingroup$ @ymb1 like most engines of the day, there were no independent fuel lines to the cylinders. Air and fuel (and oil) were mixed upstream of the engine in a (fairly rudimentary) "carburettor" and entered the crankcase through the centre of the hollow crankshaft. The mixture was then distributed from the crankcase to the cylinders. The (stationary) crankshaft had quite a large diameter at the rear end, as it had to support the whole engine. $\endgroup$
    – Level River St
    Jun 25, 2016 at 12:56
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    $\begingroup$ @ymb1 en.wikipedia.org/wiki/Rotary_engine#/media/… shows the mixture going into the engine through the stationary crankshaft. The rotating joint is between crankase and crankshaft. I suppose you could get a leak there. Fortunately the vacuum in the crankcase is not particularly high, and the worst that could happen in case of a leak would be a little more air would enter. The only mystery is the ignition system. $\endgroup$
    – Level River St
    Jun 25, 2016 at 13:18
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It is oil.

When the engine is cold, the piston rings don't quite seal the cylinder and oil leaks past and pools in the bottom cylinders. If the engine is started with oil above the pistons (looking from the centre), severe damage can occur as oil is not compressible, causing an hydraulic lock, and the piston can be prevented from rising all the way to the top of the cylinder.

"Pulling the prop through" enables the oil to drain back past the rings into the crankcase. What you can't see is that, since it's a rotary, the pistons rotate around the crankcase as the prop is pulled. If the engine has been resting for a long time, the spark plugs might also be removed from the cylinders at the bottom to enable oil to drain out too.

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