Nikkor 105mm f2.5 gets clean

The second of my camera cleaning posts is about the Nikkor 105mm f2.5 pre-AI lens.  This was one the great lenses of the seventies.  It is a longish portrait length or a short telephoto and is pretty fast.  The out of focus renders beautifully with this lens.

This particular example has sat in a camera case for the last 20 years.  It was grimy, gritty and stiff but the glass was clear, without any fungus.  The aperture was stiff and appeared to be sticky as if it had oily blades except they didn't actually look oily.  I was pretty sure I was going to have to get inside it and clean them.

The first thing I realized as I looked this lens over was that the mount has no screws!

On the left is a Nikon Series E 50mm f1.8.  On the right, the 105.  Note the lack of screws on the silver lens mount.  Ordinarily the mount can be unscrewed from the lens revealing the back end of the lens allowing access to the aperture mechanism.  Not so this lens.  I was going to have to go in from the front.

The focus ring is secured to the center helicoid by three small, black, round head slotted screws.  These early Nikkors use slotted screws throughout.  The later AI and AIS lenses tend to use more JIS (which resemble a Phillips or cross point) screws.  As a caution to the adventurous, JIS and Phillips are NOT the same.  Phillips screwdrivers will quickly strip and ruin JIS screws.  Guess how I know that.  JIS screwdrivers are available (after some searching) and are the only safe tool for JIS screws.

Slotted screws require attention as well in that the size of the screwdriver is important.  It should be carefully selected to match the width of the screw head as closely as possible in order to avoid stripping the heads.  I have a fairly wide selection of screwdrivers which I've accumulated over the years and can usually find a good match.

Removing the focus ring revealed the first bit of evidence that I am not the first person to open up this lens.  Carefully scribed straight lines matching the inner and center helicoids to the outer helicoid (and mount).  These were not here when the lens was new.  This is one of the tricks that techs use to ensure that they can reassemble the helicoids properly.  I've done this myself.  I've also NOT done it and learned the hard way that there are about a dozen ways to fit helicoids together that are incorrect and only ONE right way.  These are a good thing, as now I know that I can strip the lens down completely and have a pretty good chance of getting it back together again, working.

Next to come off was this chrome index ring.  It is secured by three chrome slotted screws.  

Removing that revealed a great deal of oil.  This is not a good sign.  This paint is NOT glossy, that is a solid layer of oil which has seeped out the the helicoid.  This is bad because it can migrate into the aperture mechanism which is supposed to stay dry AND it attracts dirt.

At this point I've wiped down the main lens body with paper towel moistened with lighter fluid.  This shot shows a couple of interesting things.  One is that dirt I was talking about in the slot above the aperture ring.  Yuck.  Another is the continued scribe line.  More evidence. Also in this shot you can see the three helicoids (visible in that vertical slot) and the key (in the same slot).  The way the mechanism works is that the focus ring turns the center helicoid (the small ring almost to the top of the lens in this view.  The mount is attached to the camera body so the force of the center ring turning pushes against the key in the slot (the small chrome rectangle at the bottom of the slot).  This forces the inner helicoid to rotate within the center at the same time the center rotates within the outer.  Since the threads are opposite the inner helicoid moves out, away from the lens mount and presto!   Focusing.

This is the same area as above except the focus ring (center helicoid) has been rotated.  It has moved away from the mount while at the same time the inner helicoid (with the chrome part attached) has rotated within the center, also moving forward, away from the mount.  You can see that the key has reached the top of the slot.  These helicoids are full of old, dirty grease and grit and really need to be completely flushed and re-greased for this lens to work correctly.

At this point I've removed the key by means of screws hiding in the holes near the base of the slot and I've unscrewed the center helicoid (and the entire lens assembly) from the mount assembly.  Point of no return here, as now I've disengaged the helicoids.   Also of note is that little, chrome screw to the right of the inner helicoid.  It and two like it hold the aperture mechanism in place.  That narrow part of the lens body actually unscrews from the main lens body now, giving access to the aperture mechanism.

Now comes the scary part, and the part of which I have no photos because I was concentrating so hard on not screwing it up, I forgot to take any.  That black ring with the silver nut on it slides out the top.  The black slotted ring underneath it also slides up, freeing nine tiny, thin, metal aperture blades from each other and the remainder of the mechanism.  They all need to be cleaned with lighter fluid, along with both of the rings, then dried and carefully re-assembled, one atop the other, delicately so as not bend the extremely thin metal, then carefully slid back into place in the lens body.  Exciting stuff.

These two photos show the focus ring (left), index ring(right) and...

the main lens body without the lens mount (top left), inner aperture linkage ring (top right), center helicoid (bottom left), inner helicoid (center) and aperture ring (bottom right).

These are all aluminum and therefore safe to wash in hot, soapy water in the sink.  The toothbrush is used to ensure that all the old grease and grit is removed from every thread and every slot.  It is really convenient that all the moving parts can come apart like this and allow a complete refresh of the lens.  Great design.

In the immortal words of every shop manual ever written, assembly proceeds in the opposite order of disassembly.  Really, though, with the markings already on the lens and a couple of scribbled notes (like the aperture ring is 9 rotations, the inner aperture linkage is 10 rotations) re-assembly was pretty painless.  I re-lubed the helicoids with my own special mixture of greases.

The lens mount is nice and clean, as is the rear element.

That beautiful piece of glass now resides in a smoothly functioning refreshed lens body.

As you can see, all the dirt has been flushed from every crevice.  The lens mounts and focuses easily and is nicely damped.  Beautiful.

All told I spent about five hours disassembling, cleaning and re-assembling this lens.  At my normal hourly rate that is about 5 times what the lens is worth.  Obviously, I do it for love, not money.


  1. John I'm curious if you'd be kind enough to share what your mixture of greases is. What I've been using comes out correct for pre-ai lenses like these but is certainly stiffer than the latter AI-S era focusing. Oddly enough I just reworked a beat up copy of this same exact lens with a later serial number and it has the classic five slotted screws in the lens mount and different internal construction regarding linkage to the aperture. Strange.

    1. Darin,
      I usually start out with Super Lube (check Amazon). If that's too thick and the lens is hard to focus I'll add a tiny drop of sewing machine oil or gun oil and mix well. It seems to last for a long time without migrating anywhere. I've got lots of greases of various kinds out in the shop but most are too heavy for helicoids. Super Lube is pretty darn light and works well for Nikkors. I also have a very light clear oil for air tools that seems to work pretty well when mixed with the Super Lube (tiny drop of oil) when very thin, light lube is called for. The challenge with the helicoids is getting enough grease (you don't want aluminum to aluminum bare contact) without being too thick to focus as well as no migration (to muck up your aperture blades) and no outgassing (to muck up your glass).

  2. Thanks John, just recently I've been using white lithium CRC grease that comes in a can on amazon. Seems to be light enough, which as you know is rare. The only downside is it needs to be sprayed into a jar and set overnight to let the carrying agents evaporate. With no oil, hopefully no out-gassing. Thanks again for the response. That lens, being a "tick mark" lens is actually worth more than you might guess.

    1. Yeah, now that you mention it, it is worth more than I assumed. I was doing this for a friend and had I realized what it was worth I might not have opened it up. :)
      Good thing I didn't slip with a screwdriver...

  3. Well it looks like you did it justice!

  4. Hi John,

    I have an old nikkor-P 105 1:2.5 with a focus ring that appears to have run out of grease or has some dirt undearneath. Have you by any chance taken apart such a lens? what is the easiest way to get the focus ring off?

    Also I have a 24mm pre AI that won't focus to infinity. Any suggestions on how to correct that?

    1. On the 105 2.5 - removing the focus ring is usually pretty simple, usually just three screws around the perimeter. Sadly, getting the ring off doesn't solve the problem of old grease. The way to solve that problem is with complete tear-down and re-lube. That is risky if you haven't done it before mostly because there is one way to re-assemble it where everything will still work and many ways where it won't.
      Your 24mm lens is an example of this. It was probably disassembled and the correct position of the helicoids not noted. When is was reassembled it was out of position and won't focus to infinity. Trial and error is often the best way to get those back together in working order.
      Helicoids consist of several nested threads. Unlike say, a bolt, which has one thread that goes all the way end-to-end, lens helicoids may have five, six, eight threads which are distributed around the circumference. This means that for each set (and there are usually two, sometimes more (Nikor 50mm f1.4) there are six or more ways to put the halves together, only one of which is the correct way.

    2. Sounds like the helicoid is off for the 24mm, this might give you an idea about how to work on the Nikkor-N 24mm f/2.8 pre AI

  5. Hi there
    I have the same 105 and stripped it down as both the focu and apeture was jammed. Got the focus moving smoothly but still having trouble now with the apeture ring connecting. Any suggestions?

  6. Thankyou!

    You saved me over 3 hours on this!


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