because life would be boring without a few setbacks... now and then... right?

Uh, is that a crack? It looks like a crack.

 I do not believe this belt tensioner is supposed to be at an angle with respect to the rest of the arm:

um, those are new since yesterday

Uh-oh, that doesn't look right!

 If only I had device, machine, or tool that can make complex plastic parts easily... d'oh!

 The best part is, I ran another test print without realizing this was busted, and the print completed.  Was run at 125% speed too, just for yucks.  I'm not saying it was pretty... O_O

 I can probably buy another one or someone's claimed unbreakable upgrade (which will be glass-  or carbon-filled, but printed, and therefore quite breakable) or I could machine one from aluminum.  I guess there's other materials I could use, but out of everything I have on hand, aluminum seems like the best choice.

 The next issue is that aluminum is heavier than whatever unfilled garbage resin Creality™ used for this part.  It's a stressed part; why was it not glass-filled nylon?  And that matters because of how this printer's Z axis works; from one end only.  The design works best if the wheels never wear at all, and the far end weighs nothing at all. <_<

 I was so naive, thinking I was nearly ready - with just some tuning and probably a bunch of filament-drying - to start printing for real.  Silly old man.  Silly, SILLY old man.

PS: machine has not been moved in years.  Belt was not over-tightened.  Well now, it's probably a tad under-tightened.  I'm frelling grumpy about this, TBH.  I think I need a break again, I feel like I'm constantly being sabotaged by invisible fairies and ninjas.  Ninja fairies, even!

UPDATE: I ordered a metal upgrade unit.  For $11, I couldn't be arsed to make one.

 This also leaves me with a spare bearing and pulley for what should be a high-wear area... if I were printing a lot. 

The blue knob is trash.  I have enough bakelite knobs, I certainly don't need any crappy PVC knobs.

 Belt tensioning is done sonically with a belt-tensioning phone app (easiest) or by calculation and any free audio measurement app.