Originally Posted by
Trakhak
Ovalizing of the bottom of a head tube is not particularly common but not unheard of, especially for 1" headtubes of bikes ridden at speed (happened to one of my bikes after a couple of years of East Coast singletrack, which is much rougher on bikes than smooth fire trails). In any event, the OP says that this sounds like the correct explanation.
To the OP: I used Loctite bearing retaining compound, which is way up there in strength among their adhesives. I pulled the stem out, used a head tube cup removal tool to pop out the bottom cup (or maybe it just fell out because of the ovalizing; I did the repair about 30 years ago, so I don't remember), painted the cup with enough compound to fill the ovalized gap, and used a headset installation tool to clamp the top and bottom cups in place. I think I left the glue to dry for about 24 hours before I reassembled the bike.
I'm going to check more thoroughly this weekend. It's possible that I didn't do a good job of putting the lock nut and the threaded top race correctly. I am going to adjust that more and see what else I can learn from the process. The idea of putting on Loctite... I don't know that I know enough to try to do that. Last resort, I will take it to a bike shop, but hoping to figure things out myself.