cleaning lens - metal flakes when putting back together

Hi, I cleaned a lens twice (Zeiss Flek) and both times, when putting it back together, metal flakes drop down into the internal elements when screwing the thing back together. Is there a way to prevent this? I cleaned around the rings and brushed the threads to no avail.

I'm guessing aluminum/aluminum?
Try waxing the threads before reassembling them.
Rub the male thread against a candle or piece of paraffin/ski wax/floor wax. Some hard wax, not paste.

OK, thanks...sounds good to me.

In case it really is the thread, than before using wax, polish the the thread with finest grinding paper. Sometimes the peak of a threading is a bit rough.