While walking home on a particularly unfortunate day at the end of 2013, I had my Leica M4-P in my hand with my Summicron attached. Handling the camera as I usually do, I suddenly felt my lens pop off the camera body (I still don’t know how that happened). Thankfully, because the lens landed on the hood, it bore the brunt of the damage, and despite bouncing on the sidewalk twice, it seemed none worse for the mishap. Back home, I realized that a rough spot had developed on the lens focusing, which annoyed me. Thinking that I can somehow ‘bend’ it back in shape, I gave the old lens a good flex, using the focusing handle (the part on which you put your index finger to focus the lens) for leverage. Little did I know that the handle was not a contiguous piece of metal molded with the focusing ring, but is instead made of plastic (PLASTIC!! What were you thinking Leica?!) held in place with a mounting screw. So now the little annoyance that is the focusing roughness exploded into an all-out repair. I wasn’t able to find too much information about the repair, but decided to give it a shot. I present to you my repair procedure.
*Disclaimer: Try this at your own risk!
We start with the lens before disassembly. This particular lens has a helical assembly and an optical assembly. We will only work with the former, so be careful not to touch any glass surface. The back end of the optical unit has a region of exposed brass, which is the rangefinder coupling surface. The M-mount is held to the rest of the camera with 6 screws. Unscrewing them allows you to easily remove the M-mount (chrome part). Generally, it is a good idea to take as many photos as possible of all features as they will come in handy during reassembly.
Once that is done you will be faced with a pretty featureless bottom, with a metal retaining ring mounted flush against the lens body. Focusing to infinity while holding the front element up, then focusing close allows you to separate this ring from the lens body. I placed a tape to index the position of the ring (it is pretty useless, though). You will see two holes into which the protrusions from the retaining ring will be inserted during reassembly. Notice that there is now a little sliding tab that is held to the lens with two screws. You will find that when you try to take the focusing helical off (counterclockwise) this little tab will get in the way, since it sits in a groove on the optical assembly.
If you remove the two screws you will end up with the tab. Notice the two small mounting holes on the helical assembly, which is at the same angular position as the focusing handle.
With the sliding tab off, you can now freely unscrew helical assembly from the optical assembly. A word of caution: The rearmost element on the optical assembly can rotate, and it is best not to touch this at all after the sliding tab comes off. Getting it displaced will require a few tries to get the rangefinder coupling interface back in place. It may also open up the lens, exposing it to the problem of internal dust. Unscrew the helical assembly very slowly, and note the position at which it comes off. I used the aperture indicating dot as a reference and marked the corresponding location on the helical with tape. Three screws now held the inner helical part to the outer part. The inner part is the one to which the focusing handle is attached. Even with the screws off, the two parts do not come off easily. Be patient. Note also the position of the handle (inner part) relative to the red dot (outer part). You can see that the focusing handle itself has two plastic indexing tabs, as well as a mounting hole for the mounting screw. I unscrewed the whole thing, and re-glued the indexing tabs and the mounting screw with epoxy and let it set for 30 mins. This prevents solvent/chemical fumes from being passed into the lens element which can be harmful.
An additional drop of glue holds the mounting screw securely.
A lint-free lens paper soaked in IPA is used to remove residual grease (I applied some wood grease previously. Too thick!!), which is replaced with Helimax-XP helical grease. This grease is still a little viscous for my tastes (NLGI grade 1 grease). It does however work well at a broad range of temperatures (-40 degC to + 149 degC), important to prevent solvent deposition on optical components. Microtools sells an HG-10 or S-10 grease that is NLGI grade 00, which is significantly softer but has been out of stock for a while. Reassembly will be easy if you follow my instructions, taking lots of pictures along the way. My lens has a nice, sturdy handle (better than before, methinks!) and also a smooth, if a smidgen stiff, focus. Good luck!