This lens is mechanically very different from its successor, the f/3.5 AI-S but this is a positive given the AI-S version's more complicated setup - the f/3.5 AI follows a pattern found in other AI lenses, such as the Nikkor 50mm f/2 AI, and in the end was reasonably simple to service. The only tricky problem with the f/3.5 AI appears to be at least one of the rear mount screws is always glued which can lead to stripped screws.
Helicoids
The focus grip is rubber but since your lens could be from 1977 the rubber may appear very hard. Remove the rubber using a toothpick going around the edges. Once the rubber focus grip is removed, you can spin off a focus ring collar which gives access to the set screw on the lens objective retaining collar. If the focus ring collar is sealed, apply accetone to the hole under the lip and accetone all around the joining seam and leave before attempting to spin off with rubber tools.
collars and rubber focus grip in the background
Once the collars are gone, we can pull the single peice objective from the barrel body.
On the rear, we can remove the mount and the aperture ring comes on easy. The aperture fork floats freely once the rear mount is lifted and you can pick it out - it has a removable long screw (like the 50mm f/2 AI) that couples with the aperture ring but its not necessary to remove that screw - just pick out the entire aperture fork ring.
From the front of the lens unscrew the 3x screws holding the focus ring and remove.
At this point make 2x reference marks against infinity: one of the rear section (the rear helicoid) below the chrome grip and one on the thin ring (the central helicoid).
When done with your infinity reference marks, remove chrome grip - the grip slides off towards the front. We are now able to access the helicoid key but take note of how it looks and gap between the central and rear helicoids. When rematting, you need to get to this position.
infinity marked on the barrel - the mark on central helicoid is obscured
We want to get some measurement references:with the helicoid positions in a known "good" state determine the correct "height" of the central helicoid at infinity. At infinity the top of the inner helicoid to the under side of central helicoid lip measures about 17.5mm. This should be the position when remated. We will note that you can only rotate it about 10 degrees past infinity before the inner helicoid's aluminium housing hits the stop on the rear helicoid. Pay attention to this as it helps you determinme correct positioning when reassembled - make a mark if you wish but make sure its distinguisable from your infinity mark.
To access the 2x screws to unseat the helicoid key, you must rotate the central helicoid as if focusing towards MFD - notice the helicoid key has a "lip" that points to the rear of the lens. Take note of how this looks and relative positions of the central helicoid so can have an easy reference when reassembling.
helicoids have moved away from infinity to allow access to helicoid key screws
To get the helicoid key out the way, rotate the front aluminium peice whilst ensuring the central helicoid doesn't move too much to avoid unexpected separation. At this point you could also fully collapse the central helicoid to mark it against the rear block to give you move reference points upon remating. The central helicoid will rotate, from infinity, 1 + 3/4 turns to get to full collapsed.
With the helicoid key gone, you can turn the central helicoid past the infinity position.
NB: all the screws required to separate the helicoids are located in front of the aperture ring - this means that it will be possible to service the helicoids without removing the rear mount. Not requiring the removal of the mount is a positive given the often glued rear mount screws/likelihood of stripped rear mount screws. Reinserting the helicoid key is not too cumbersome and reinserting the lens objective to catch the aperture fork and aperture activation fork simply requires the aperture ring at f/22 and the aperture and actuation tabs on the objective set accordingly.
Seperate the Helicoids
Hold the rear helicoid stationary with the infinity mark visible and slow turn the central helicoid whilst applying gentle tugging pressure. As soon as you feel the central helicoid starting to pull away/jitter its almost about to separate. When the central helicoid separtes, mark the position on the central helicoid relative to the infinity mark on the rear helicoid.
separation mark made under the lip of the central helicoid
The inner helicoid is seperated through the central helicoid as if it were being pulled through the front of the lens. Same procedure as above - hold the central helicoid stationary with the infinity reference mark visible and slowly turning the inner helicoid whilst gently pulling away. Mark the seperation point on the inner helicoid in relation to the infinity reference mark on the central helicoid - on this copy it lines up nicely with the right edge of the objective cuopling slot.
inner/central helicoid separation position - mark clearly on the inner helicoid once lifted off
Now seperated, go and clean with the usual tissues, lighter fluid, dish soap, kitchen grease remover (Cilit Bang kitchen grease remover works well once you've done with lighter fluid!) and final clean with more lighter fluid and IPA. When ready use a VERY thin layer of soft grease, like NPC BC-13A, on the chrome helicoid surfaces only - for the matting black surfaces I just use the dry brush/whatever is left after applying to the chrome part and wipe over the black helicoid parts.
Remate and you're good. Do note if you get the central helicoid distance wrong, you will be caught out when attaching the focus ring - the focus ring physically sits at a given position and the metal shim will require the central helicoid at a specific height so that the focus ring can be secured.
With the lens objective reseated, add the objective collar and verify/adjust infinity focus before adding the focus ring collar.
Optics
The 11 / 8 optics are relatively easy to get to although they are sealed with lacquer and need acentone / IPA to disolve the seals to open up. There are no good references for the optical formula except the following from mir.com.my.
We do need a curved lens spanner or similar to access the main internal block of elements.
logical block 1 - elements 1-3
To access the 1st element we need some accentone/IPA to disolve any seal behind the front ridged retaining ring - this has quite fine threads so care is needed when reassembling. We can spin this off using a rubber cone tool whilst its gripped by a larger cone tool. The 3rd element are sealed behind a (in this case a brass) retaining ring that is sealed with lacquer - accetone/IPA and spin off for access.
The second block requires a curved lens spanner to unseat - this houses the bulk of the lens elements.
logical block 2
element 4
element 4 - can be removed from its housing from the rear
With the cap removed we can access the rest of the elements in the this block.
underside of the housing
Do note the size of the housing and the number of elements that it houses - these elements are relatively small so special care is needed when handling, especially when determining orientation of removed elements.
The rear element housing is sealed with lacquer from underneath.
rear element 11 on the right
Final Considerations
Whlist the f/3.5 AI is the older sibling of the AI/AI-S lenses sharing the same optical formula, the internal mechanical complications of the AI-S makes the AI lens much more desirable when we consider servicing and not even considering the longer focus throw (100 vs 70 degrees). Externally both the f/3.5 AI and AI-S lens look near identical, with the AI lens' aperture ring having an orange f/22 marking (instead of the much more common blue marking of other AI aperture rings) - however the AI lens's tell-tale ADR f/22 is still white and f/3.5 marking is in green whereas the AI-S lens' equivalent is white.
Nikkor 20mm f/3.5 AI - notice the colours of lettering on aperture ring as easiest external distinguishing factor between AI vs AI-S
This lens isn't the best when paired with modern higher resolution sensors or compared and lenses with it's distortion and softer rendering across the frame until we get to about f/5.6 its still a fun lens particularly shooting into the sun or with the sun in the frame. If you have a choice between the two AI/AI-S, I'd very much consider the older AI version if you intend to service it.
random workbench, taken through the serviced Nikkor 20mm f/3.5 AI (on DX) @ f/4 near MFD/~0.3m
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