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Welcome to FTE. Be sure and scroll further down our forum listing and stop in at our 7.3 diesel section. Lot's of good info about the 7.3 down there.
In answer to your question. I'm not sure. Did you loose yours? If I had to guess I'd say 3/16" ID. Maybe 1/4" but from what I remember it didn't seem that big.
I used some medical oxygen tubing. The female end of the tubing is a good friction fit without needing any clamps to hold it in place. I cut the tubing so it hangs down low enough that I can place it into a bottle and catch the fuel I drain out. It's been in place for years without any problem