Help calculating piston rod angle

[Philippe Hurbain]

Yep, that's the same selection info tool I mentioned. Trouble is, I can't figure out quite how to use its features to get the precise measurements I need, because I don't yet have all three of the points I'm interested in. Looking at my diagram, I can't just take the angle of ABC, because A and B will have moved up the Y axis to a new position, which I don't know. So the angle I need is A'B'C, and first I need to find A' (and by extension, B').

Alternatively, if I knew the angle, I could get the new Y position perfectly easily by using part snapping.

Yes, you need to do a bit of trigonometry here...
   
We want to turn the piston arm so that its tip is at the distance between red pin and initial axis of arm (vertical here). Since I put the gear on the origin, this distance is directly pin X coordinates (13.856), but you can also derive it in the general case from LDCad measurements as sin(16.10°)*49.96. Note that the blue parts are just helpers parts to get the right measurements points.
Once we get this value, we calculate piston arm rotation angle as arcsin(13.856/60)=13.35°. Rotate the arm by this value and use snapping on red pin to correctly position the pistion arm...