The Rotax manual tells us to turn the engine prior to starting only with
reference to verifying the oil level in the reservoir tank (i.e. with empty
crankcase). The manual also explains how the turning brings the oil back to
the tank: Compressed air is leaking past the piston rings into the
crankcase, and this overpressure in the crankcase pushes the oil back to the
tank.
This leakage of compressed air past the piston rings will also blow away
some oil deposited on the cylinder walls during the down stroke, while the
scraping of the rings removes some oil both in the down and up stroke.
I doubt that there is any meaningful buildup of oil pressure (i.e. enough
for lubrication) during this short-lasting and slow hand cranking. If there
were some pressure with resulting oil flow, it would mean that oil is
flowing back into the very crankcase that we try to drain, wouldn't it?
I have attended the Rotax maintenance course first in Denmark, later in
Norway. The very experienced service agent for Scandinavia both times
recommended doing this turning-for-oil-level-check immediately after
stopping the engine - not prior to starting it. The reason is that the
engine is easier to turn when warm, and the oil has lower viscosity - it
therefore flows easier back to the tank. This difference is noticeable on
cold winter days.
Risk of oil lock in a cylinder with our type of engine/lubricating system:
I do not know.
A different matter altogether is whether to turn the engine after it has
been preserved with special oil, e.g. for winter storage. Turning should
then NOT be done, as it will scrape/blow preservation oil off the cylinder
walls.
Regards,
Svein
LN-SKJ
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