A couple of thoughts on what makes jam

And not the nice kind of jam you spread on bread.

I would really like to know why my extruder jams. Otherwise I'll just go on replacing things and having more trouble - and I've sure had enough trouble already.

I had a stripping back when I used the old bolt. At around the same time, I left the extruder at 220 for 20 minutes without extruding, moved the printer to a less stable surface, added the fan, and eventually changed the bolt.

I've been able to clean it enough to get a half-way decent print, but it slowly extrudes less and less again.

I had to relevel the bed, it has become almost a full millimeter "higher" - or the extruder head lower/longer. I did one successful print after the overheating incident without having to relevel, though, so at least it didn't immediately become longer.

One possibility is that there is now a little gap between the top of the hotend and the extruder. When doing prints with backpressure, this gap gets filled slowly either with stripped-off dust or squeezed-in filament.

Alternatively, there's something further down that causes slow filament buildup. I can atomic out the buildup, but not whatever is causing it.

I would get a newer hotend, but they're all so much shorter that I'd need a different fan shroud, which I can't just print. Obviously I need a printer to make parts for my printer.

I don't get why hotends are being made shorter at the cost of increased complexity. Having 1cm more extrusion height doesn't help much if you can't print because your hotend overheats. A long-necked hotend ought to have an easier time keeping the hot end hot and the cool end cool, but instead it's all cooling ribs and extra fans and fancy materials, just to get a little extra vertical space that 99% of people won't ever need.