ok, you said sledge hammer.
remove the gear.
inspect the crank AND key CLOSELY for even a tiny flaw and fix it with a fine file if you see one.
Take some 280 grit wet/dry sandpaper and water or wd40 and sand the inside of the gear heavily . this will smooth any little machining burs or ridges etc.
put the gear in the oven at 300 f for 15 minutes.
put assembly oil on the crank.
install the gear with ease.
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