KF
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It's imperative to do it to check for hydraulicing (oil in the cylinders) on a radial or inverted engine. Not to do so risks destroying the engine.
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Hydraulicing is a situation when excessive oil is present in the cylinder bore. It happens easily on inverted engines and the lower cylinders of radial engines because after shut down it is easy for oil in the crank case to run down the cylinder walls, seep past the piston rings, and pool in the cyclinder heads.
If there is sufficient oil in the heads, and the engine is started, the piston on the compression stroke will come up against an incompressible layer of oil and will stop dead. This usually results in a bent or broken con rod, and the condition is called hydraulic lock.
The results of 'lock' can be more insious than instant engine destruction - if the lock is relatively minor, the engine may continue to run OK with a slightly bent con rod. At some indeterminate time in the future, this damaged con rod will fail, and the engine will eat itself - usually when at full throttle, like take off for instance.
If you pull the prop through by hand before starting, and there is a 'lock', you will be unable to turn the engine over and may have to remove the spark plugs to get the oil out of the cylinder(s). But see next paragraph....
The Yak 52's Vendeneyev radial is particularly prone to locking, and great care must be taken to avoid the condition. On this engine, it is posible to pull it through, get no 'lock', but still suffer one on start up. This is becuase the inlet ports extend beyond the level of the inside of the cylinder heads. On the lower 3 cyclinders, oil can pool in the 'U-bend' at the bottom of the inlet pipes, and remain there during manual pull-through. On start up, the induction gases blow the oil into the cylinder on the inlet stroke, and you get a lock on the compression stroke. To prevent this, there are drain plugs for these 3 inlet pipes - if you even suspect there might be oil there, you have to unscrew the drain plugs, which will allow it to drain out.
There are, I beleive, mods availabale to automatically do this, without having to manually remove the drain plugs. If I still had a Yak share, I would insist on having this mod fitted.
SSD