How to perfect align rotation? How can i align the rotation of two objects perfectly?
I have this cylinder between two blocks and i want to perfect align the piston end (orange) to the cylinder (rotation).
You first need to correctly realign the object's orientation.
To realign the object orientation with its face orientation, select a face of your object in Edit mode, create a custom orientation in the Transform Orientations panel:

In the Options panel (top right of 3D view) activate Transform &gt; Affect Only &gt; Origins:

In the header menu click on Object &gt; Transform &gt; Align To Transform Orientation:

Deactivate Only Origins, do an AltR, now your cylinder is aligned with the Global orientation again, it should be easy to align with the other cylinder.