With a -6 drain line to the crankcase, air that was initially inside the gearbox is mixed with the oil and this mixture is forced out the drain by an equal volume of pure oil coming in.  Eventually all the air is eliminated and the gearbox is completely filled with oil.  An RD (Tracy Crook) redrive fitted with a -10 (5/8”) id drain and a vent from the upper part of the redrive to the upper part of the crankcase will not fill up with oil and only have crankcase pressure inside.

Steve Boese 



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Mat,

                Oil into the redrive is needed mainly for lubrication and also heat transfer.  Very little is needed as lubrication needs little with oils these days.  It appears that In most cases the redrive actually fills and does not drain readily.  Why I do not know.  This puts pressure on the seals back and front, which suggests it may be beneficial to hold both seals in with small screws to prevent them getting pushed out.  Screws are good insurance as in flight a detached seal could ruin a good day.

Neil.