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10-16-2021, 05:38 AM   #1
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Jupiter 21m screw sizes

Hi all,

I am the new owner of a 93' Jupiter 21m 200mm f/4 lens.
Unfortunately, this lens has been taken apart at some point in its life and whomever did that had their IQ in the negative.

The lens is missing a lot of screws and two of them are completely rounded off.

As it sits, this lens will work, the focus is smooth, however, there are many issues:

the aperture is completely free, if you rotate the lens clockwise you will move the aperture at f22, if you move the lens counterclockwise the lens will move back to f4.

Can someone please tell me the size of the screws used on this lens?

I want to use this lens a lot, mainly because it has almost perfect glass inside, even in its sorry state, it takes great photos!

Thanks in advance!

10-16-2021, 09:40 AM   #2
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Wow, a 93 foot 21 metre lens? That's huge. ;-)

The various screws are likely metric sizes, maybe check these folks:
Jupiter 21M 200mm F4.0 Lens Reviews - Russian and Zenitar Lenses - Pentax Lens Review Database
10-16-2021, 10:55 AM   #3
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Welcome to the Pentax Forums!

The screws are likely metric diameter, but the standard for the thread pitch would be the larger question. A lens repair shop or a vendor of fasteners might be able to help.

An easier solution might be to buy a Jupiter 21M that is complete and in working condition, given that it is hard to say whether the threads of the empty holes are still good.


Steve
10-16-2021, 11:18 AM   #4
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QuoteOriginally posted by ProfessorBuzz Quote
Wow, a 93 foot 21 metre lens? That's huge. ;-)





QuoteQuote:
The various screws are likely metric sizes, maybe check these folks:
Jupiter 21M 200mm F4.0 Lens Reviews - Russian and Zenitar Lenses - Pentax Lens Review Database
Metric they may be, but some distant memories of equipment of eastern European manufacture suggest that they may not be "standard" metric you'll need at least the thread pitch as well as the diameter and hope that the angles aren't too far out!

10-16-2021, 01:39 PM   #5
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It may just be a matter of poor tolerances but if the thread pitch are non-standard than there are no ready sources. All the catalogs of metric screws of the 2mm and under diameter range never list more than one pitch if they even list a pitch at all.
10-16-2021, 02:34 PM   #6
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QuoteOriginally posted by ProfessorBuzz Quote
Wow, a 93 foot 21 metre lens? That's huge. ;-)

The various screws are likely metric sizes, maybe check these folks:
Jupiter 21M 200mm F4.0 Lens Reviews - Russian and Zenitar Lenses - Pentax Lens Review Database
93' as 1993 which is the lens date of manufacture.
I already checked many of those threads already.

Thanks for your help!

---------- Post added 10-16-21 at 02:36 PM ----------

QuoteOriginally posted by stevebrot Quote
Welcome to the Pentax Forums!

The screws are likely metric diameter, but the standard for the thread pitch would be the larger question. A lens repair shop or a vendor of fasteners might be able to help.

An easier solution might be to buy a Jupiter 21M that is complete and in working condition, given that it is hard to say whether the threads of the empty holes are still good.


Steve
A complete lens in my area would go for around 80 bucks, I paid for mine around 30. The lens is 100% Complete but it has missing screws.
To add insult to injury, the two rounded screws are what's holding the mount of the lens attached to the body, so I can't remove that thus, i can't fix the focus ring.

At this point, I am considering just getting a small drill bit and drill out the rounded screws and just insert bigger screws which will tap their own new thread. Would that work?
Drilling and using different screws means going from screws that have a thickness of 1.27mm to screws that are around 2mm. Is that too much of a size jump?

---------- Post added 10-16-21 at 02:40 PM ----------

QuoteOriginally posted by Not a Number Quote
It may just be a matter of poor tolerances but if the thread pitch are non-standard than there are no ready sources. All the catalogs of metric screws of the 2mm and under diameter range never list more than one pitch if they even list a pitch at all.
Given that this lens is not such a high tolerance system, could I get a way with just drilling out the rounded screws and insert bigger screws? Also, for the missing screws, same question, could I drill the threads out and use bigger screws? I am talking about enlarging the threads which are 1.27mm to around 2mm.
10-16-2021, 04:01 PM   #7
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If available to you you might try something like this before doing any drilling. I bought one of these to fix sunglasses and there are a lot of different sizes in there.

10-16-2021, 04:11 PM   #8
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A tap is pretty cheap. I think I would order some 2mm screws, a tap, maybe the correct sized drill bit for the tap and take a chance. If you are super cheap and have extra screws, you can file the threads on one screw along the axis of the screw. That screw makes an OK tap in wood and could maybe work in aluminum. Back it out often and use oil to make cutting easier. You don't have a lot of leverage with this method. Amazon says taps are a few dollars so it's probably worth getting the right tool.
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