Detach the helicoid from the front element. It requires a lot of turns... at first it looks like it is only rotating, instead of unscrewing!
Now the most fragile and soft part of the lens is exposed: this is the center lens assembly, locked in place by a conical ring. GOTCHA! This ring in my case loosened, causing that horrible rattle and a strange optical effect (because the center element was dislodged from its optimal position).
Unscrew and remove the conical ring to access the center element; to unscrew it only friction is required; it is large enough to be unscrewed with bare hands. But remember NOT TO TOUCH THE LENSES!
CAREFULLY estract the center element (it a relatively big and heavy block of optical glass... DON'T DROP IT AND DON'T LAY IT AGAINST HARD SURFACES!!!) and gently clean it and the rear of the front lens with the blower. We are at the bottom end, at last!
Place the center element back in place, and tighten the conical ring.
Now CLEAN EVERYTHING AGAIN; everything must be shining. The all-metal parts can be washed in soapy warm water, brushing them with a discardable toothbrush until perfectly clean. Then dry them thorougly, better with a not too warm hairdryer. Then it is time to remount everything, following the reverse order, and reapplying a reasonable amount of molybdenum bisulfite grease. (I choose this kind of grease because it gives a mild sensation of friction, adheres to metals, resists to heat and provides protection against oxidation, humidity and corrosion. And if it works for high load bearings, it surely works for focus helicoids...)
On the focus helicoid apply grease BY HAND in the right quantity to fill the threads, but no more than that. It will give a nice 'braking' feeling when focusing... After remounting, wipe away the excess of grease squished out. But leave a thin film on the outside rim... it acts as a barrier against dust. We don't want to have to open again the lens because some particle of dust enters the helicoid and starts scratching...
The remounting is pure boredom, if you marked everything. The only tricky part is to reinsert the focus ring; it takes a steady hand and some patience. DO NOT FORCE THE ENTERING ON THE HELICOID! When it is well aligned and parallel it will screw in like a charm.
And now let's take a look at the final result....
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