I believe the risk is small unless you fill all the way to the top of the added 'filler neck," when spillage could become a problem. Diesel fuel on asphalt is like black ice to a motorcyclist.
The coefficient of thermal expansion of diesel fuel is 0.00083 (on average depending on blend/additives) per/˚C, and not including the numerous biodiesel configurations.
Like all liquids, diesel fuel expands slightly in volume as its temperature increases. The coefficient of thermal expansion measures the rate of the expansion over an increase in temperature. A typical value of the coefficient of thermal expansion for diesel fuel is 0.00083 per degree Celsius (0.00046 per degree Fahrenheit). Using this value, 1.000 gallon of diesel fuel at - 7 °C (20°F) will expand to 1.037 gallons at 38°C (100°F). (About 4.7 ounces, over an 80°F increase in temperature.)
This means if your tank has a capacity rated at 25 gallons it is possible for it to expand up to 25.925 gallons in the described scenario. Meanwhile, a tank designed to hold 25 gallons of fuel must be designed to be at 95% capacity maximum, which means a tank advertised to hold 25 gallons will actually be a 26.31579 gallon tank to allow 5% for expansion. That means that in the above scenario there would be 0.39079 gallons of empty space left after expansion.
I intend to add the Extendafill necks to my Seneca soon, because a few of our destinations are "just short" of our range considering our generator usage, and it would be great to just fill up at my regular place when I get home.
I'm going to measure the height of the top of my tanks, then will score a line inside the filler necks before installing, so that I can know when I'm "full" and won't risk inhibiting the functionality of my vents and tank equalization system.
Just my 2¢, discounted today to 0¢, so take it for exactly what it's worth and understand it's not advice, just my own opinion. Ultimately, the worst that can happen is that it flips upside down and kills us all.
