Very well done that man.
Hans, I know exactly how you feel, when you bring to life something that has been made out of inanimate lumps of metal. You want to run around letting the world know you have succeeded.
For your little cylinder face problem, it will be most probably one of two things.
The first is the pin sticking out of the cylinder is not square to the face, in which case, a gentle tweaking to get it square will cure it.
The favourite in my opinion, would be the big end being either drilled off square, of being forced off square. Try slackening off the locknut on the big end/piston rod joint, and see if it cures the problem. If it does, maybe a bit of fine filing on the locknut seating face will bring it back to sitting square when the nut is tightened. Otherwise it might be a hole squaring up exercise.
Well done again, and hope the grin lasts a long time.
John