Also worth mentioning the headset top nut may be a little smaller ID than the steerer. So try the stem in the steerer without the top nut as well. Unfortunately, steel headset top nuts are sometimes hardened. You may have to sand it or grind it, because it may just dull a file or reamer.
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Owner & co-founder, Cycles René Hubris. Unfortunately attaching questionable braze-ons to perfectly good frames since about 2015. With style.