Does anyone know what it is?
I can get the pipe in 5mm, 6.5mm and 8mm.

If you want the PIPE, I'm sure some one will givvvve it to you

The stock fuel hose is 7mm, which is not a common size. Some will use 1/4" (6.35mm) and others 5/16" (7.94mm). The 5/16" may seem to be a little loose but I have not a problem, it actually fits very well. Make sure you use clamps.

From the selector valve to the carb tee 5/16" with clamps works ok.  I tried using 5/16 from the two tank taps to the valve and couldn't make it work; the two hoses are too thick to both fit onto the tank chicken together.  I ended up doing those two runs in 1/4" instead, and then they JUST got past each other.  The 1/4" is quite tight and you have to wrestle it on.  All of the above with Goodyear automotive hose.  I also added a clamp to the vacuum hose where it hits the valve body.  That cured a vacuum leak that caused a fuel starvation issue at open throttle.

Jeffrey
'96 500ET

You can fit the thin wall 5/16" clear/transluscent Poly fuel tubing side by side on the fuel valve without a problem.

That thin wall sounds a better idea than the thick Goodyear automotive hose, but that thick stuff was all my local auto parts place had handy.