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During runup and takeoff roll the manometer (tapped into the crankcase vent
about the mid-point of its length) will read zero until plane has about 120
knots indicated airspeed.  At 120 to 200 knots the manometer reads 4" to 5"
with a short spike up to 14" to 20" when power is reduced in the pattern.

This symptom suggests that airspeed is pressurizing the case and pushing oil
out. Check the Oil filler cap for a proper gasket and for proper sealing of
the gasket.