Inexpectadly, I realized that the back focus ring of my lens Fujinon 16x5.5 was free because the thread hole whre the a little screw ins, was broken with a rank that runs from edge to edge, so it can't keep firm the general focus! Could somebody suggest me where this problem could be fix? I attach two jpegs.
Thanks

I would imagine a $19 tap an die set would fix that and put in the next size larger bolt. But wait till someone else chimes in, in case that might be a stupid idea.... in other words, don't blame me if you foul it up worse. :)

Re-tapping the thread to a larger diameter won't work as the ring is cracked across.

The ring itself likely has to be replaced.

Is the ring metal or plastic?

Even if glued back together it will require dismantling from the lens body otherwise you'll risk gluing up the lens itself. If you have to go that far then replacing the part altogether is best.

To make it work right now, maybe cut a thin strip of gaffer tape. Lay tape over the ring around the lens to stop it moving once you set backfocus. Or buy water-based white bathroom sealer. Set the backfocus. Put a few spots of bathroom sealer over the gap between the front of the ring and the body. Don't push the sealer into the joint.

Let these spots set hard. They will hold the ring firm enough but can be cut away later as they don't go rock hard. If a spot moves in deep between the ring and the body and you want to get it out, break open a bic single blade shaver and take the blade out. Use scizzors to cut a point on the blade. Wrap gaffer around to protect fingers where you hold it. This blade is thin enough to cut out any spill of sealer which gets in the gap and goes hard.

Thanks guys for your advises booth are useful. I think to replace the ring (it is plastic made) could be the best way. Do you know some place where use to fix lenses and possibly made this repair?

I have a 16x5.5 Stock Lens available is case you would like to buy one second hand.
It's as brand new.