One of my wife's bicycle pedals fell apart. The plastic part degraded. However, I was under the mistaken impression that it was a simple thing to order a replacement online. Not so simple.
First off, there are two measurements; The thread width, and the wrench width. Ah, a comedy of errors ensued. Plus I didn't have the correct wrench at first. My cresent wrenches wouldn't work because the part of the plastic made access too narrow. Even with the correct wrench, I ended up stripping the pedal. Part of this was because I was wrenching it in the wrong direction. The threads are left or right, and they have mirror images in terms of the threading.
I went to a motorcycle/bicycle shop for help and they started in on it. Couldn't make any progress. I finally picked up the tool I wanted but didn't have, a correctly sized vice pliers. Then, I went to work. Leaning over the back of the bicycle seat, I had my right foot on the right pedal to keep it steady, then I put the vice pliers on the left wrench end of the pedal and started to move it in the correct direction (counter-clockwise).
Part of what made this successful, though it took a good three minutes of applied pressure, was that I had the right tool, was in the correct position, and I knew that I was wrenching it in the correct direction. Ah, sweet success.