If you haven't started don't just muscle it off. I'd loosen the extractor bolt and make sure it moves freely, IIRC about a turn before it starts pushing against the retailer. It isn't easy but do try and dribble a little oil in microscopic gap, if you turn the extractor bolt back and forth to try and suck it in that may help. These can be a bear to get off if they are dry and the bolt head binds against the retainer. There usually a brash 'bearing' or even a Teflon washer between the two to aide it in sliding.
I always loved that crank but sadly it was never offered in 172.5
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk