What you could do is (a) create another Style that looks like the relevant Chapter title Style, except for any numbering; (b) move the overflow text into a paragraph using that Style; then (c) press Ctrl-Alt-Enter at the end of the Chapter title paragraph to join the two using what Word calls a 'Style Separator'. This way only the shorter part of the title will be included in the header.

Instead of (c), you could just format the Chapter title's paragraph break as hidden text. That has the advantage of working on Macs, where the 'Style Separator' isn't supported.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]