Hi Nobby,
Long time no hear.
The standard "C" oil tank holds 6 pints for the twin & the Comet.
Following this logic, the "D" tank holds 5 pints, the same tank being used on the twins and the few singles made (Victor)
I have a redundant "D" tank you could use for measurements if you want.
If you decide to copy, let me know cause i´ve been looking at getting one made in stainless (but slightly enlarged in width)
The enlarged tank i´ve got on the D at the moment is on it´s last legs (it is only thin guage plate, and internal rust through condensation has taken its toll)