Tessar 1:4,5 F=25cm *Fliegertruppe* lens repair.

[Cheffalo]

This lens: http://www.flickr.com/photos/cheffal...2806/lightbox/

There are cleaning marks (residues from a wet cloth) on pretty much all the lenses inside. I managed to unscrew the front element from the aperture part and could clean the back lenses but the front element needs to be disassembled too. It can't be done. The threads will move about 4 millimetres and then stuck. There has been a stop screw but it's drilled away, leaving aluminium particles inside the element, between the lenses. What's the trick here? Anyone here with similar experiences?

[Dan Fromm]

Squirt some lapping compound into the hole where the set screw was, work the element back and forth. Getting the thing to unscrew will take hours.

How do I know? I once sent a TTH telephoto that was stuck in (wouldn't unscrew from) an Agiflite mount to Steve Grimes with a request to cut the mount off. Instead he handed the thing to the new hire, Adam Dau, who now owns the business, and told him what I just told you. Adam still remembers the exercise, is still unhappy about it.

[Cheffalo]

Oh dear. And if it doesn't work I'll have some lapping compound between the glasses too.. LOL

I tried some lubricant/corrosion solvent and that made the difference with the four millimetres, not more. I guess the lapping will work slowly forward in both ways. Guess it will take a couple of days.

Thank you for the info, Dan!

[Steven Tribe]

+1 for using lapping compound (Valve grinding compound) for any brass threads. It removes oxide build-up and any slight distortions. Just back and forward and measure your progress from time to time. Particuarly for useful for flange threads that are imperial/metric combinations!

Getting this on your glass is not a problem as long as you "wash" it off, rather than rubbing it away.

[Cheffalo]

It's aluminium, another soft metal so I guess it would work the same.
I made a self portrait test with the lens, stopped down to f8, and can't see anything from the stains on the inner element in the negative so I will wait with this procedure until I know for sure I have the right compound at hand. I was thinking of a cleaning paste I have based on fine ground white silica flour, would it be something similar to that?

[pasiasty]

Sometimes alcohol helps in case of aluminium. I mean a few drops to the thread (you may also take a few glasses orally).

[Cheffalo]

Ha ha, well it may help to solve all the stuff I've put in there by now
The thing is that it's running quite freely back and forth, these 4-5 millimetres, and it stops very sudden. I have a feeling there's some little piece of something stuck in the threads or broken threads.

[Cheffalo]

Gah! Finally! my hands are sore and will be full of blisters tomorrow.. I used the polish compound, diluted with water, and poured it into the thread while working it back and forth. Nothing happened. I kept on and nothing happened, thinking I should give it a go for a while just to be certain. Then all of a sudden I could work it a couple of millimetres more and from there it was just hard work and stubbornness.
Now I will only find a way to treat those threads to prevent this from happening next time I have to take it apart.

Thanks for all the help!