Please don't hesitate to ask me any doubt regarding this conversion.
Ziuko Nikon 1
This is the Olympus OM-System Zuiko 21/3.5 lens.
Zuiko Nikon 2
It has three screws at the bayonet base, let's remove them.
Zuiko Nikon 3
And just lift the mount.
Zuiko Nikon 4
Now make a spring like this one from a piece of 3.50mm
Zuiko Nikon 5
We have to unscrew this screw just the necessary to pass the spring under it.
Zuiko Nikon 6
This way, here I've used a tweezers and the plastic tool.
Zuiko Nikon 7
Now, with the hook, pass the other side around this aperture lever.

Again, the plastic tool was useful to keep the aperture lever on position.
Zuiko Nikon 8
Ready, it is important to check if the iris close until the smaller position, if not, it means  the spring too long.

It is a good idea to drop a small quantity of nail varnish on that screw, so it will not get loose. Please let the varnish dry completely before mounting the lens on the camera:  the smell must disappear.
Zuiko Nikon 9
I've painted the white metal screws with my flat black marker from Micro-tools.
Zuiko Nikon 10
All the screws are black now.
Zuiko Nikon 11
Now install the Leitax mount using the provided screws, the orange line goes in front the scale center.
Zuiko Nikon 12
And you can use this Zuiko lens on any Nikon F-mount or Pentax camera.
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