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Relubing a 3M-5CA 500mm mirror lens
Posted By: GibbyTheMole, 02-11-2012, 08:43 AM

Hey all.

Just posting this in case someone else has a 3M-5CA with a stiff focusing ring due to old grease. I re-lubed it today & it was pretty simple. Definitely worth the effort. In case someone else would like to tackle it, here's what you'll need:

1) Lighter fluid
2) Cotton swabs
3) Clean paper towels or cloths
4) 1.4mm straight-blade jeweler's screwdriver (tiny!)
5) White lithium grease (or silicone grease)
6) Grease pencil or small piece of masking tape

Here's how I did it:

1) Using the 1.4mm screwdriver, remove the three screws just under the scale marks on the focusing ring. The screws are near the 5 foot, 10 foot, and infinity marks. Be especially careful you don't lose these. They are smaller than the head of a pin. If you drop one on the carpet, you'll never see it again. I would hold the lens over a bowl to catch the screw when removing.

2) Once the screws are out, unscrew the front half of the lens counter-clockwise. You may have to unscrew it 'til it stops, turn the numbered scale ring clockwise to move the focus stop, unscrew the front of lens a little more, move the scale ring again, etc., until you get enough space between the ring and the rear of the lens to allow the front to be completely unscrewed.

3) Take note of where the focus arrow on the top of the lens is, and using the grease pencil or a small piece of masking tape, mark the spot on the front lens barrel lined up with the focusing arrow where the lens finally comes apart. The threads are triple-start threads, and when you put it back together you'll need to line up the threads at the exact same point to thread it back on without damage.

4) Once the lens is in two pieces, clean the bulk of the old grease off the threads with a dry paper towel. Once you have most of it off, finish up with cotton swabs moistened with lighter fluid. You'll need several. I held the lens with the opening facing down to prevent fluid from running into the open lens. It's a messy job, so be careful you don't touch the mirror with the swab while you're in there. It doesn't have to be absolutely perfect, just get as much of the old grease off as you can. I finished up with a dry cotton swab to get rid of any lighter fluid/grease residue.

5) Check to be sure there are no cotton fibers or paper towel remnants on any of the threads & once you do that, you can use your finger to apply a thin, even coat of new grease to the threads on the front half of the lens.

6) Lining up the marks to align the threads, screw it back together. If you start to thread it back on and it doesn't want to go, DON'T FORCE IT. Back it off try it again until it threads on smoothly.

7) Put the lens on the camera & focus it on an object at infinity. (The moon is a good choice.) Line up the infinity mark on the scale ring with the arrow and replace the screws. (Once I had the infinity focus point aligned with the infinity mark on the ring, I used a piece of scotch tape to stick the ring to the lens barrel so it wouldn't move when I replaced the screws. I also used a tiny blob of grease on the screwdriver to stick the screw to the bit, then carefully threaded them in. The screws are by far the biggest pain involved, since they are ridiculously tiny.)

Annnnnd... You're done!

Replacing the old grease made a HUGE difference. It focuses like new, and it was relatively simple to do. It took more time to type out how to do it than to do the job itself. I should've taken photos of the process, but I didn't think of it 'til the job was done.

Cheers & good luck,
Bobbo :-)
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