I had this problem with a set of cheap Shimano cranks. I also had a cheap no-name crank extractor tool that was always a bit iffy, never seemed to fit the crank threads very well, so I bought a genuine Shimano crank extractor.
Before resorting to more destructive methods I thought I'd give it a go with the new crank extractor and hey presto, off they came. The Shimano tool obviously fit better, enough to fit in what remained of the stripped threads.
Before resorting to more destructive methods I thought I'd give it a go with the new crank extractor and hey presto, off they came. The Shimano tool obviously fit better, enough to fit in what remained of the stripped threads.