I seem to recall the F1 is 20mm, and is larger than most other Honda's. Even the F2 is smaller. The 750 K5/6 and F1 use the same tap, but they are hard to find at a decent price, and second hand ones usually need the filter and sealing washer renewing, only available as a kit with no change from £20 for the genuine kit. Get an accurate measure across the outlet of your tank, just in case it's been changed for an F2 at some point in it's life.
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