Arrived promptly, thank you FLO for the usual good service. The rings do what you'd expect and fit fine. As already commented by others:
Do, Lubricate the threads before use.
Do, make sure that a pair of rubber "Marigolds" other brands are available! are to hand as gripping the small rings if they do lock up is difficult.
Don't over tighten unless you have no intention of undoing them.
If they do pickup try heating the external ring (internal thread) in a shallow pool of hot water, not recommended if a camera or lens is attached.

Excellent service but for the first time after purchasing many items over the past 8 yrs from FLO this is the first time a product has been less than 100% satisfactory. The rings come stacked together and several were thread bound out of the box but finally separated with aid of rubber gloves. When snugged up found they became loose again but nippimg up a little bit tighter become thread bound. Iwould risk using them in between other spacers but nervous using next to oag, filter wheel or camera in case they remain seized as too narrow to use strap wrench and quite flimsy so would probably distort using pipe grips/wrench. Obviously the other review for these rings is a positive 5 star so maybe I've just been unlucky with this set.

These spacers are ideal to get that back focus accurate - a good range of sizes
product is well made and threads have been formed well within tolerances (nice secure fit without binding)
the 'blacking' is a little bit shiny to start with this should dull down with being outside (no reflection perceived)
Ideal when changing the cameras to suit target
excellent service by FLO as always