Disassembly of the f/4 EL-Nikkor Enlarging Lens
The original 4 element metal barrel f/4 lens has never been considered a top performer compared to it's 6 element cousins. I think that's a mistake, but it does make them very inexpensive on the used market. At normal print sizes, say, 8x10 inches, I'm hard pressed to see real differences in sharpness, even at the edges. The lens does need to be in tip-top condition, and that may require cleaning or recementing of elements.
If you're not somewhat experienced with camera and lens repair, you should probably stop right here. Take the lens to a good shop for a CLA (clean-lube-adjust). If you elect to proceed, remember that I'm not responsible for whatever you do to the lens. In addition, THINK! The instructions may not match your version of the lens, or may not be appropriate to the work that needs to be done.
For internal cleaning or repair, the lens needs to be disassembled to some degree. Often, simply removing the front element as in the next paragraph will suffice. In general you shouldn't disassemble any further than absolutely necessary to get the work done.
The front element is held by a slotted retaining ring but you should NOT attempt to remove this. At least not yet. The entire front assembly can be unscrewed from the body. Grip the front lip (using rubber if necessary) and unscrew it. Hold the body, NOT the aperture ring, as you don't want to stress stops or blades. The front may be tight, but it will unscrew.
Don't remove the aperture setting ring unless you need to service that part of the lens. Watch out for small springs and balls used for the detent.
The flat black rear surface is actually a screw-in cap. It can be removed with finger pressure, or with a small piece of rubber to improve your grip. Watch out for the lens element surface.
When the rear cap is removed, you'll see a reverse ring that holds the rear elements in place. This too can be unscrewed with friction, though it has a side hole if a suitable tool is available. The rear elements can now be removed to a piece of lens tissue. Note the orientation.
All the surfaces are now accessible for cleaning, and reassembly is just the reverse of the above procedure. I haven't found it necessary to remove the front element, but if you do, you'll need a suitable spanner. Note that the hallmark of a good repair person is that they leave no evidence of service or repair. That means that tools must fit perfectly- screwdrivers, spanners, etc.
Inspect the cemented elements for separation. This may be an irregular fine line near the OD of the elements. It can be very subtle. Look through the elements at various light sources, both specular and diffuse. If you find a separation, it will have to be repaired if the lens is to perform as well as it should.
The original cement was balsam. You can heat the lens to soften it, or soak it in solvent. A slow gentle heating with a hot air gun has been effective. When the elements are apart, clean them thoroughly with alcohol to remove the balsam.
A modern UV cure optical cement like Norland #61 should be used for repair. The elements should be placed on a piece of lens tissue, flattest side down, and machinists v-blocks can be used to insure good centering. Centering is critical to lens performance, so be sure your method is sound before applying and curing the adhesive. See the S.K. Grimes site for additional recementing info. This is a one time deal, so be sure the elements are perfectly clean and properly centered. If there is any doubt, STOP. Solve the problem and/or obtain assistance.
I've had good success curing UV adhesives with a quartz-halogen lamp, as they tend to be rich in UV. It will take several hours at a close distance (watch out for excessive heat). A good UV source made for this purpose might take ten seconds to a minute. Bright sunlight is also effective, though still quite slow.
When the lens is reassembled it will still be a much-maligned 4 element lens, but you should find the print quality far exceeds common wisdom about 4 element lenses.