Penetrating oil! Thank you, it's off now. I feel like a total dumbass, I must have just been burned after working on this all day...Usual remedies should help - penetrating oil, tapping, careful prying.
Funny you should mention that... I managed to chip the sprocket in two places, on the outer lip and the inner edge of one of the teeth. Should I replace it now?If everything else fails - smash it and get a new one (mostly kidding).