If you loosen the two bolts that clamp the stem to the steerer tube you might be able to lift the cap off, it depends if the star nut is still engaged in anyway with the inside of the steerer.
@andyp started
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If you loosen the two bolts that clamp the stem to the steerer tube you might be able to lift the cap off, it depends if the star nut is still engaged in anyway with the inside of the steerer.