I got 15 years out of a Shimano UN-26. Not too shabby. I've gotten a lot more miles out of a cup/cone Shimano 600 BB though and it's still going strong.

I got 15 years out of a Shimano UN-26. Not too shabby. I've gotten a lot more miles out of a cup/cone Shimano 600 BB though and it's still going strong.

Huh? It's fairly obvious that the pedals on the OP's bike were never properly tightened at some point, so any stress that caused thread damage was from force being applied was because it was loose, and it eventually wore.

I've always had a problem with the thread orientation of pedals. Not a problem on the bench, but why they chose to reverse the threads on one side or the other, Think about it. If the pedals on either/both crank arms were the same, there would be no more failare rates than what we see currently.

The orientation on pedal spindle threads are designed to tighten the spindle to the crank arm if the bearing fails, and it will, but why? I'd hope that one would recognize a bad pedal bearing long before it was "tight enough" to unscrew the spindle from the crank arm.

There is absolutely no reason to reverse the pedal spindle threads on one side of the crank vs. the other. When you install pedals you more likely use a wrench with a lever of about a foot long. Hopefully you've torqued it properly. Assuming the pedal bearing freezes, the lever in place that might "un-loosen" it might be what- 1 1/2- 2"?

That being said- The damage to the crank arms shown in the OP's fliks ae repairable with ether a heli-coil or a product similar to a Keen-sert. It is nowhere beyond the damage that either method would not affect a "like new" repair. Can't get it done at your LBS? Go to a shop where they know what's happening. Cost will most likely be in the $20/ each side neighborhood.

No, the use of a left-hand thread in the left pedal would actually serve to loosen the pedal if the bearing were to seize - not tighten it. But that's not the reason for the left-hand thread. In properly functioning pedals, the bearing friction is insignificant compared to the force due to precession as the rider exerts a downward force on the pedal as it is turning. The torque on the pedal axle causes slight movement of the threads at the axle end inside the crank thread. See the following under "Attachment":
https://en.wikipedia.org/wiki/Bicycle_pedal

This precession movement would tend to loosen a right-hand threaded pedal on the left side of the bicycle - something I experienced first hand on my tandem when I installed a child's crankset on it which was reversed from the normal orientation (my daughter's chainring needed to be on the left side to connect with my cross-over chainring in front). The result was that the pedals kept loosening even after I used considerable leverage to attach them and even though it was only the relatively minor force of a 4-year-old pedaling that made them loosen up.

Lock-tite solved my problem, but the problem of left pedals coming loose was recognized fairly early in the history of the bicycle. It's been reported that the solution of using a left-hand thread for the left pedal was developed by a couple bicycle mechanics in Dayton, Ohio named Wilbur and Orville, who are also known for some other inventions.

my experience is that older shimano sq taper cartridge bbs are bullet proof. current plastic cup ones are still decent