A sticking valve is the most common I've heard of, at least that's what got me. It's possible that the push rod was already slightly bent when the engine was rebuilt. If a guide was a bit snug it may have been just enough to finish the job.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.