Simple question. Are there different pedal thread sizes for the varying types and manufacturers of cranks? I have a pair of older SPD pedals that I couldn't get onto one of my old bikes with an even older Campy SR crank, early 1980s. It wouldn't thread all the way on. I put those same pedals on a different bike from the same era, with a Zeus crank, and it worked. But I've also swapped pedals between those bikes in the past without problem.
What I'm looking to do is put these pedals on my current ride, a Kuota CF bike with an FSA SLK crank, not as a permanent addition but to allow someone to give it a test ride. She rides her current bike with SPDs on it. This way we can swap bikes back and forth without having to swap pedals on the road. (She's also drop-dead gorgeous and could beat the pants off me in our indoor cycling class and about 20 years younger.) I don't want to start threading them in only to find myself ruining the soft crank thread.
I just hope my shoes with the SPD cleats haven't fallen apart. They look to be in better shape than my current road shoes, but you never know if some glue has given up after all the years.