Topic: petcock fitting size

[famoussas1]

Got a new (old) tank and what i thought my previous tank was (75 cb550 tank with locking fuel door) the petcock will not fit on what is the exact looking tank besides the petcock fitting size.  my old tank was around 18mm and what looks like a 20mm on the tank I have now.  can anyone clarify?  did honda enlarge the tank thread size for larger size petcock?

[TwoTired]

For US 1977 models, the fuel valve bung was made smaller and uses a different fuel valve.  The bung changed size from about 3/4 for early and 5/8 for late.
The in tank filter design changed as well, and doesn't swap between early/late models.
The door over cap tank design/style was used on all the Cb550F models and the 77-78 Cb550K models (US).

Cheers,