[.a.t.]

Search for "riffler" a type of small file.
Short cutting area, curved on a longer bar- usually have two ends; can be triangular, rounded, or square; of course you would need the triangular type.

More knowledge! Thank you, Don.

[Robot camera]

That looks like an aluminium thread. Find an old filter, stepping ring, lens hood, whatever you can in that thread size. It should be brass, as it needs to be harder than the aluminium. Cut five or six slots into it at right angles to the threads with a junior hacksaw or similar. The cuts need to go just past the bottom of the threads. You have now made a tap.

1) Protect the front element before you start, from grease and little bits of aluminium.

2) To use it, apply a smear of grease to the tap and gently turn it into the lens thread. Easy does it; it's a tiny thread. Make sure the tap is not cross threaded.

3)Turn it in as far as you can, then back it off a quarter of a turn. Then try again. Each time when you can't turn it in any more, back it off a quarter of a turn. DO NOT FORCE IT. Repeat.

4) Every so often, remove the tap completely and clean it up. Go back to 2). Be patient.

The way it works is that the cut thread edges of the tap slice off the aluminium that's in the wrong place. Any bits of aluminium swarf will collect in the hacksawed grooves. I've done this trick on motorcycle threads when I haven't got a tap of the right size, though usually it is with a steel bolt into aluminium, not with brass which isn't so hard. Good luck.

[.a.t.]

. . .
Good luck.

Thanks, Robot. That sounds like a clever method. If I had a proper brass piece to spare, I'd give it a try.