Hi Steve, is the enlarging of the oil return necessary?
I thought you first tried enlarging the return line expecting it to improve draining, only to find no improvement until the vent line was added.
Jeff

Jeff,
At this point I have only 3 hr of flight time on the gearbox with the modified drain and vent.  I have not observed a significant effect with respect to power absorption or heat generation.

Connecting the vent line to the foot of the rotor housing was ineffective even though this location was above the oil level.  Connecting the vent line to the area of the oil filler opening at the top of the center iron allowed the gearbox to drain as intended.

The most troublesome part of the modification was enlarging the oil return to 1/2" NPT in the rotor housing foot.

Steve Boese