Repairing a Schneider Curtagon 35mm f/4 Lens

The Schneider Curtagon 35mm f/4 seems to be a pretty rare lens. Web searches don't come up with much (any?) information. Everything I was able to find was only about other versions, a "PA" version (tilt-shift) or Retina versions (silver, with DKL mount). On ebay, today, I find a small number of these listed - but in the history ("sold") there are none.
According to a list of Schneider-Kreuznach serial numbers, my lens was made around 1959. It has the M42 mount and a preset aperture with 8 blades. It has a nice weight and feels very well made.

My lens came with a pretty stiff focus, so I thought, I'd give it a try to disassemble and clean it. Although I couldn't find any tutorials, these older German lenses are usually relatively easy to disassemble - so I gave it a try.

Unfortunately, I did not fully succeed: At the end, I was not able to separate the last two pieces to get full access to everything. But I was able to clean and relube most of the lens internals, and able to make a little workaround for the last piece. This gave a huge improvement of the focusing.

I'm starting by removing the nameplate with a rubber tool.

With a lens spanner, I remove the ring that holds the front glass piece.

And I take out the front glass with a suction tool.

This gives access to the ring holding the next glass element.

Now, I can access the aperture from the front.

I continue from the rear side, taking out 3 grub screws that hold the metal ring.

A lens spanner in the outer two holes lets me remove the rear glass.

Now the aperture is accessible from both sides.

Now the front piece can be taken out (I had to wiggle quite a lot!).

This screw is setting the limits to the focus ring.

After loosening this screw, I was able to turn the focus ring until the inner helicoid became free - and further rotating the inner helicoid made it free. Note where exactly it becomes free - and where it exists!

Now, I don't remember what this screw was for (sorry....).

By removing these 3 screws, you can take off the focus ring.

After removing these 3 screws, I was able to remove the back ring.
 

But at this point I was stuck. I have no idea how to separate the inner rotating ring (to which the focus ring is connected) from the rest. Older German lenses (Steinheil, Zeiss Jena, Meyer-Optik) have a second (fine) helicoid for this purpose. But for this lens, it seems that this is not rotating using a helicoid. So, I could not take it off to clean and relube it. But I was able to flush some lighter fluid in between the two pieces, which improved it a lot and it made rotating the focus ring much easier and smoother.

During my attempts to separate the ring, I also tried to remove the helicoid keys. But this did not help, and it was not easy to screw them back on. So I strongly recommend not to try this!!

Due to my failure at this point, I stopped taking pictures of the reassembly of the lens. But since I documented the disassembly in detail, I hope that these pictures are still helpful. 
If you figure out how to separate that last piece, please leave a comment!

In any case, my lens is now in great shape - and it's fun to take pictures with it!

More of my lens repair tutorials can be found here.