How full is full... Aztec fuel tanks

[Coopere]

I have been pondering this for a while, when I fill my inboard main tanks on my aztec right full, and I mean up to the bottom of the filler ring on the filler neck (so the fuel level touches the the bottom of where the fuel cap seats) and then cap the tank and go for a flight, I find that both thanks, left and right, expel some fuel, I am NOT burning of the inboards on this test, I suspect the fuel is venting out the fuel cap and then going out the fuel vent tubes at the filler

The tanks seem to expel fuel until the fuel level is about 3/4 of an inch to 1 inch below the metal fuel neck ring. Then they stop and stay at that level indefinitely

It has been this way since ive purchased the aircraft over the last 4 years

I know the bladders are not leaking, if I fuel it full as described above on the ground and then let the aircraft sit on the ground for a week no drips and the fuel level is exactly where I left it if I check the tank

I suspect this is relatively normal, ive never drained the inboards completely so I do not know if the quantity is exactly 35 gallons, or if i'm short a bit due to this, I have not done fuel planning so exacting that this matters as I would never fly the aircraft so low on fuel that this would cause a fuel outage

any thoughts?

 

[JimICS2452]

 

[philiplane]