The 24mm f/2.8 AI-S lens is reasonably simple to disassemble and teardown with the only complication being the CRC unit at the rear.
Youtube channel mikeno62's Kenneth has two highly detailed videos related to the 24mm f/2.8 which shows the cleaning of lens elements and re-greasing of focusing helicoids which provided the basis for this disassembly/teardown.
WARNING/DISCLAIMER: any actions you take upon the information presented here is strictly at your own risk and we will not be liable for any loses and damages in connection to this information
As noted the optical formula has remain constant for my iterations of the Nikkor 24mm f/2.8 - the cross section below can be used as a reference to us when reassembling.
(c) Nikon - The Thousand and One Nights #86 - optical cross section
In the AI-S version, the 9 elements in 7 groups are arranged in 3 optical blocks; the front (3 elements) group, the middle (2 elements) in front of the iris and the rear block (4 elements).
Nikkor 24mm f/2.8 AI-S Dissassembly
Only standard lens maintanence tools are required:- lens spanner
- JIS-0 screw driver
- small flat ~1.2mm head screw driver
Rear elements
Firstly remove the bayonet mount by applying accetone/nail varnish remover to the 3x cross head JIS screws and then unscrewing - following this remove the aperture tab (lift up and out - this drives the iris mechanism when the aperture ring is turned) and the aperture ring.
If you simply want access to the rear elements without removing the entire optical rear assembly/objective, I found that you MUST remove the brass CRC tab that fits into the focusing fork BUT you MUST mark where the the brass tab sits so you can return it to the original position; if this tab remains seated, it is highly likely that unscrewing the rear element will bend the CRC tab - this is very bad news as it means focusing will always be off. Below left, we can see the brass tab and right we see the tab removed. In Kenneth's YouTube video, he is able to use a rubber tool to loosen and remove the entire rear optical block but my copy was very stubborn and was not able to unscrew the rear optical block.
the CRC brass tab is deformed - someone before me has obviously been in the lens
Notice the fork that the tab sits in - also notice the 2 white marks on the optical block/CRC that are aligned with the lens at infinity: this was there (from the factory??) to ensure alignment for correct focus. If there are no markings, you should make them yourself.
With the brass tab removed a pointed lens spanner can be used to remove the lens elements in the rear block.
Optical Assembly/Objective
The entire optical assembly is removed from the front and is secured by a retaining ring and a collar. The outter retaining ring needs some good rubber grips to unscrew which will then give you access to the internal collar; the collar has a set screw holding it in place that can be accessed by setting focus to min focus distance - loosen the set screw and twist off the collar.
.With the collar removed, you can remove the objective but if you look down at the lens you will see a brass ring held by 3x cross head screws which secure the focusing ring. Removing/reattaching the focus ring is simple as the lens barrel has hard the near and infinity focus stops that mate against the grooves on the focus ring.
To remove the objective you can gently work it out of the barrel - there is a slot with what looks like a screw (its a guide only) and you can slide the objective out. With the CRC brass tab removed this is not too difficult, but if you are removing from the front and have NOT remove the rear bayonet mount, then you will need to pay attention to the CRC brass tab since it will need to mate back into the CRC fork.
Front Elements
The front optical block contains 3 elements and can needs a slotted lens spanner to remove; once removed, the front element can be removed by unscrewing the front retaining ring with rubber grips - the front element will simply drop out.
Flipping the removed optical you can start removing the elements with a pointed lens spanner; the 2 elements are each held in place by their own retaining ring.
Middle Elements
The middle optical block sits in front of the iris, contains 2 elements and is unscrewed using a pointed lens spanner; the elements are secured by a top level retaining ring holding the outer most element - when this ring and first element is removed, another spacer separates and holds the other element which can be popped out using gravity or lens sucker.
Almost done
At this stage, we've stripped the lens down as much as we need - the only things that are left to disassemble are the aperture mechanism and focusing helicoids which don't need attention unless there's something wrong (oil or re-greasing respectively) for which the youtube links at the top can advise.
Reassembling Gotchas
There are not too many issues with reassembling the lens elements as we have the lens cross section and nearly all of the elements can only fit in one way or they have lens spanner notches that face one direction / have marks that indicate you have opened.So the lens elements should be easy.
Reinserting the lens assembly / objective into the lens barrel can be tricky:
Without CRC brass tab attached
This is the easy case - align the large screw on the lens assembly with the notch at the front of the barrel and slide in. Do not force it too far. At the rear, with the lens focused at infinity, realign the CRC marks with focus assembly and reattach the CRC brass tab into fork and screw down. If you get this wrong you will notice that things at infinity are not quite in focus.
objective alignment notch (on chrome) visible at the 12o'clock position
With CRC brass tab attached
This is a little more tricky as you will need to ensure the objective's CRC brass tab is aligned with the CRC fork at the rear of the lens barrel. The easiest way to do this is to focus lens at infinity, slide the objective into the barrel and as the objective gets towards the rear, you will have to move the CRC tab so that it aligns with the CRC fork and then slide it in fully.
CRC fork (black but marked on the left), at 1o'clock, to the left of the brass helicoid key
Aperture Ring and Bayonet mount
The aperture ring slides onto the barrel and you should align the aperture ring and the focus dot at f/22. With this in place the aperture tab can be fitted - this only has one place it can go and will be moved when you turn the aperture ring.
And finally, the chrome bayonet mount can be attached - ensure that the bayonet's fork is engaged with the aperture lever (see above left image, screw driver pointing at the lever at 9o'clock) and reattach the screws: with the aperture ring set at f/22, check the aperture lever moves the iris and turning the aperture ring opens/closes the iris. With this in place, you're done.
Was it worth it?
This for me was originally an exercise in exploration, bringing out the kid that played with lego back in the day. With the right tools (JIS screwdriver!!) this was reasonably straight forward. For Nikkor manual focus lenses that would be approaching 30-40yrs old, you may need to clean out some of your lenses anyway and whilst not all lens are the same, it should give you confidence in your ability to do the job you need.
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