I found that it was quite difficult to accurately adjust the Z-opto end stop on my Prusa. Once I got to the point of having to move it up or down by 0.1mm I was oscillating between the correct point for quite a while before I got it right.So this design is for an end stop with an adjusting screw. Now I can get the setting almost right, then use the screw to fine tune it.1) Print the two parts. Clean the holes, making sure an M3 bolt will easily pass through the holes.2) Place a washer then a spring on two of the 25mm bolts. Insert the bolts through the opto. Thread the top plate onto the two bolts with the hole upwards.3) Place three M3 nuts into the nut retaining holes on the top plate, two on the bottom, one on top.4) Thread the two 25mm bolts through the top of the bottom plate and into the retaining nuts. Screw the bolts down until they are just proud of the top of the opto.5) Screw an M3 nut onto the other 25mm bolt, wind it until it is near the top of the bolt. Thread the bolt through the bottom of the bottom plate until it captures in the top nut.6) Mount the end stop on the machine and adjust it and the end stop flag so they are correctly positioned horizontally.7) Bring your z-axis down until it touches the bed at the correct height.8) Wind the middle bolt up until it touches the top plate, then wind it up a few more turns.9) Move the end stop up the bar until the opto flag seems to break the beam in the sensor.10) Home the Z axis and check the bed spacing.11) Wind the middle bolt up or down accordingly and repeat step 10/11 until it stops at the correct position.12) Wind the locking nut up into position and recheck.14) (Optional) Add cable tie to hold wire securely.15( (Optional) Celebrate!