Pre-Ai lens with no screws in mount ring

[George Mann]

Hi guys. I need to access the aperture coupling on a pre-ai lens. But it has no screws in the rear ring.

Has anyone disassembled one of these?

 

[Mamiya_Repair]

While most of the Nikkor lenses have mounts that need to be removed in order to remove the diaphragm ring, on the earlier style lenses with no screws on the mount, all you do is remove the one slotted screw on the side of the diaphragm ring and you can turn the ring to unscrew it from the mount.

 

[George Mann]

Well, I took the screw out, removed the ring and had a look at how it worked. The head of the screw disintegrated upon reinstalling it.

I managed to get the screw in enough to engage the slot, but the aperture is way off, increasingly so as I stop down.

F5.6 is F8, and between the two is F11, so I can either use stop down metering, or overexposed a stop.

Does anyone know where to find another screw for this thing?

 

[AgX]

McMaster-Carr offers down to M2.

 

[Mamiya_Repair]

To replace the diaphragm ring coupling screw in this lens, take a 1.7mm x 6mm machine screw and carefully file or grind down the end 2mm of threads to the minor diameter of the screw.

 

[shutterfinger]

https://www.metricscrews.us/index.php?main_page=index&cPath=98_298&zenid=j8u5b1q22uelvsfvgiunv0nk07

 

[George Mann]

To replace the diaphragm ring coupling screw in this lens, take a 1.7mm x 6mm machine screw and carefully file or grind down the end 2mm of threads to the minor diameter of the screw.

What is the actual size and pitch of the screw in question?

I would need a vice which I don't have access to in my campervan to perform this procedure.

 

[BradS]

reminds me of the cartoon flowchart that used to get mimeographed and passed around the cubicle farm back in the days when computers were hidden away in a separate room with raised floors and massive amounts of air conditioning

one path through the flowchart went something like this.....

"Does the damned thing work?"
No
"Did you fuck with it?"
Yes.
Dumb shit! Does anybody know?
Yes.
You poor bastard.

 

[shutterfinger]

As far as I know thee is only one thread pitch for metric screws under 2 mm.
Got a pair of vise grips handy?

 

[AgX]

 

[George Mann]

View attachment 286915

No M 1,7?

 

[AgX]

Well, that was the only table with pitches I could find in short time.

Moreover I found at a dealer in addition 1.5mm screws but no 1.7mm

 

[shutterfinger]

https://www.metricscrews.us/index.p...h=98_298_303&zenid=j8u5b1q22uelvsfvgiunv0nk07
count the number of peaks in 1mm of the existing screw or measure the distance between two peaks.
3 peaks in 1mm=1/3= .33; 4 peaks in 1mm=1/4=.25.
.35 thread pitch listed for some screws, non listed assumed to be .2.
Metric are millimeter per thread.

 

[Mamiya_Repair]

To keep things simple: The 1.7mm screw is used in about 99% of all film cameras from Japan. And 99% of them have a .35 pitch.

 

[George Mann]

To keep things simple: The 1.7mm screw is used in about 99% of all film cameras from Japan. And 99% of them have a .35 pitch.

Which would include the one in mine I take it.

Now it's just a question of which material is strong enough for the job (the original was weak)?

 

[AgX]

No M 1,7?

Seemingly it is not accidential that no M1.7 thread is appearing at that german thread list. I checked at two major sellers of micro screws in Germany and there is no trace of a 1.7 thread . Quite some 1.6 screws, a few 1.8, but no 1.7.

 

[Mamiya_Repair]

Which would include the one in mine I take it.

Now it's just a question of which material is strong enough for the job (the original was weak)?

Just to help things along,

Which would include the one in mine I take it.

Now it's just a question of which material is strong enough for the job (the original was weak)?

George, just to help things along, if you DM me with your info I will drop the correct part, ready to go, in the mail for you.