The MC18 top yoke as standard has around a 20mm spacer under it above the head race lock nut but is part of the yoke. What you might find is you have a MC21 top yoke that someone has then made up a spacer for as the '21 yoke doesn't have this spacer as the head stock is taller on the '21 than the '18.
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If I have to take the carbs off once more...