Hi friend, I am sending you a picture about, your issue.
I hope my comments, have had help you.
Answered with a tutorial: https://grabcad.com/tutorials/i-cannot-see-the-part-axis-eg-revolved-part-while-assembly
If you want to make an axis of the crankshaft you can make it with manually, you go to the sketch (choose plane with the crankshaft stand vertically) and click "project 3d element" the middle of the crankshaft, or you could do what Hugo Alanis said if you just want to know the axis temporarily to apply constraints. Ciao