Originally Posted by
cyccommute
You may have seen the threads being pulled out of an aluminum frame but I really doubt you've seen distortion of the threads on the bottom bracket itself like the one on the left side of your picture. I've removed thousands of bottom bracket shells over many decades of working on bike and the steel cup pulling apart is only a recent development. It's due to the "steel" used on extremely cheap bikes being contaminated with metals that shouldn't be there. Most likely aluminum which doesn't alloy with iron. I've seen ball bearings that are ground to dust in only a few hundred miles, spindles that are cracked and pulled apart, and many cups that have pulled apart in the frame. It often feels like the bottom bracket is cross threaded and it takes a lot...A LOT!...of effort to remove them. Oddly, the threads of the aluminum bottom bracket are often fine and a new (better) bottom bracket will thread right in without any issues.
Not sure what type of distortion you're referring to in the first sentence, is it not the same thread distortion pictured on the lower right shell? I dealt with many aluminum bikes with heavily distorted thread in the pandemic when I was buying them for cheap, refurbishing them in my garage and reselling them for good money. Many of these bikes had an incredible amount of mileage on them, yet people were willing to pay hundreds thanks to the like-new feel and 30-day warranty and unquestioned return policy (unless the bike was run-over by a vehicle). Man, I miss those summers.
I've never personally pulled thread out of a BB before, nor dealt with one thats suffered the most extreme galvanic corrosion, but a customer recently came-in with a sealed bearing race BB that was seized up really bad in an aluminum Specialized due to galvanic corrosion . They took the brake side out themselves along with the thread, but the drive side still had the shell, cup and outer race. The spline quickly stripped, and we couldn't cut through the outer race despite getting a metal blade onto a Sawzall with Mach Lube. After 5 hours, we gave up, a first in the shop's history.