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Mine is 12mm, iirc. In the old money that would be 7/16. It can be any fuelproof hose, because the upstream side of the pump is not pressurized. I have not heard of a petcock-pump hose collapsing from being too thinwalled. A thinwall hose is easier to work with in that location.
I changed mine this year, took out of sheath it said 11mm. I got 7/16 multi/biofuel and the hose said 7/16-11mm. Slid it back in sheath, put tape on end, just like factory.
When I changed mine a few years ago I went with 1/2" because I could not find ANY 12mm around anywhere. The 1/2" was really too big, and never really gripped well on the petcock. So I ended up researching hose sizes and ended up finding some -8 an hose with nylon braid that was just a little bit smaller then 1/2", and fit perfect! They have the steel braid also, but its really too stiff to clamp, and was tough to fit into the tight spaces.
1/2" is .508".
Uhhh.. sorry, but 1/2" is .500. <shrug>