I put the 13t sprocket on the wifes bike a few days ago. I figured it would be a 5 minute job cause I have an impact but that was not the case! I tried the breaker bar too but the wife stepping on the rear brake didn't keep me from turning the rear wheel. I tried a little propane heat (not much) but that didn't help either. I finally got my air hammer out and one little brrrrrp and it was instantly loose. Either my impact is getting worn out or the nut must friction lock to the retaining washer...No damage to the nut either as I hit it on the back larger nut section.
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db

And by the way...the mod makes the bike mucho better. Can't wait for the rear sprocket mod later down the road!