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Just looking at the headset it looks like a 1 1/8 inch, but when I tryed to replace the stem that came on my fork with a 1 1/8 inch it was too big, so, I assume girvins take a 1 inch stem? Is this correct?
http://www.k2bikes.com/PDF/techLibrary/1997/vector2_assy.pdfItem 28 (steerer) seems to suggest 1 1/8 and 1"