focusing helix

..i have part stripped an early preset Takumar lens (200mm f/5.6) - on reassembly i now have the aperture ring 180 degrees rotated from it's original position. I think all i need to do is 'start the helix off' in the corrrect position. But how many starting points are there? Are there only two or could there be more??

If you have completely stripped a helicoid, you of course see the number of thread ways. Otherwise you do it by try and error. If you are lucky you will be in the right thread orientation before finding out the number of ways...

Keep in mind next time to mark the orientiation of the two helicoid parts to each other the moment they come apart.
Which means doing it slowly and under slight pulling.

The wide threads usually have about three starting points, Usually it really does not matter which one you start with. On the other hand, the narrow threads have only one, You must find that. When roughly put together, test for infinity and mark well. Remember, the narrow threads have to be almost (but not quite) close to the maximum that it can be screwed into the lens body for infinity to be possible. Make tests and take your time. I wish I were there to help you. but I cannot fly a helicopter to ... Wyoming. - David Lyga

..when reassembling i tried 'clocking' the lens 'block' around a tiny bit at a time to find where it would engage freely with the helix and was surprised how reluctant it was to 'go'. I need to mark it when I get it apart again and just persist. It's quite a simple lens but the engineering looks good.