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If the input shaft has seized to the pilot bushing, perhaps putting the trans in gear and turning the driveshaft back and forth could work it free. Make sure the crankshaft isnt moving with it (which is how you would know if this was the problem).

Never heard of an input shaft seizing to a clutch hub. Maybe possible, not sure how within any realistic temps it would achieve.

A totally wrong mix of parts could possibly allow the CSC to push past the pressure plate spring and be stuck on the wrong side of it. This seems very unlikely unless something was way wrong to begin with, like wrong application parts. This would be a clearly visible condition by looking in through the little window.

Otherwise, sticky dowel pin or missed bolt.

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