Draining Fuel Tanks

[4-Shipp]

Today I drained the left tank to replace the o-rings on the sump drain. I first drained the tank thru the gaskolator and then got the rest out when I removed the sump drain. I even shook things good to try to get everything out. I then filled the tank at 2.5 gal increments to calibrate a dip stick. The "empty" tank had 1.25 inches of fuel just in front of the lateral rib below the fuel cap. When all was done it only took 167 gallons, meaning there were 5 gallons that did not drain. Obviously 2.5 gal are unusable. I'm assuming the other 2.5 gallons are usable in level flight attitude but will not drain with the tail on the ground (6.00 tires with stock B model gear). Anyone have similar experience with this? Thanks.

Bruce

[GAHorn]

Did you have your fuel selector in either Right or Left? Some fuel selectors have been reported to allow fuel transfer between tanks in the Off position.
Replacing the O-rings in the Curtis drains are not a good idea. Curtis advises against it, even tho' they once would sell the seals. (And by the way, they are not really o-rings. They are flat rings and O-rings are not reliable replacements anyway.)
The problem with Curtis type drains is that the end-cap of the valve is "button-riveted" onto the shaft. After years of vibration and allowing the valves to "snap" shut, that end button becomes weak. When it finally lets go, all your fuel drains out inflight. Quickly.
The recommended procedure is to replace the drains with new ones. Available from Aircraft Spruce. 877/477-7823.