Howdy all.

Would anyone know what the size of the rubber fuel hose / line is that enters into the stock fuel rails in our 5L's?
Is it 3/8 ?

From memory, 3/8" is close to 10mm, which is usually vaccy hoses, etc.
IIRC you'll find fuel hoses will be around 7.5 - 8mm which is close to 5/16". I've found 7.5mm to fit without a problem.
This is for an I6, but I'd think V8's would be same.
If no one replies later, I'll confirm this from the shop manual.

The other thing is what are you planning to do ??? Is this info for replacing hoses??, as they aren't a straight pull and replace with a new piece.

Nothing planned specifically but certainly interested in the diammeter and how much it can flow.