I thought there would be loads of stuff on how to do what I am attempting but I cant find anything.

How do I chop the top off this cylinder in a manner that leaves a clear sine wave all the way round. I have projected some curves and tried both circular array and rail revolve but I cant get the end points to align. Even if I could achieve this goal it still isn't clear to me how I could chop the top off. I am guessing I could rail revolve a surface to separate the top and bottom of the cylinder.

Any suggestions?

Thanks
Keith