Trick To Removing Retaining Ring?

[payasam]

ZeissFan is correct. My enlarger, for example (a Meopta), takes 75mm lenses in a flat disc but 50mm lenses in another disc which holds the lens closer to the negative.

 

[rxmd]

I'll buy some WD-40 or similar oil tomorrow. I've turned the lens with as much strength as I can muster to no avail. It seems well and truly stuck.

DON'T use WD-40 anywhere near optical machinery. It's OK for car engines but NOT for this kind of job. Around cameras it doesn't have the nickname Wanton Destruction for nothing. Given enough time, it will creep God knows where, including into the lens, into the aperture mechanism, and onto the lens elements.

In the Nikkor there should ne not a single part that needs or should get oil anywhere near it. This should be a purely mechanical operation. The retaining ring has two notches for a reason.