I've suggested that they be merged. Basically, anyone who has at least +5 in answers with cranks or crankset should be able to upvote (or downvote) the synonym.
I like crankset as the master for cranks, so I did it that way.
You can vote here: https://bicycles.stackexchange.com/tags/synonyms?filter=suggested&tab=newest
The way it currently stands, the cranks tag seems to specifically describe the crank arms. I would suggest that we create a description for crankset that is inclusive of all the pieces of the crankset and then set cranks as a synonym.
I have no opinion on whether or not we would then want to create a new crank-arm tag. I guess I'm leaning toward not doing so and just having those questions included in the crankset tag, but I don't have a strong opinion about it.