Removing front of Leitz Summitar 5cm?

[Nitrofunk]

Hi! I have a Summitar 5cm with a very stiff aperture mechanism. According to this site (http://justinlow.com/articles/repair-leica-summitar) you have to unscrew the front element to get access to the mechanism. I manage to unscrew the outer ring (the line "A" marked in the attached reference picture), but to get access to the aperture mechanism, I have to unscrew the second, slimmer ring (the one just above the aperture-numbers; marked "B" in the picture). However, I can't unscrew it - it's way too tight. Is this the right tool for it: https://www.ebay.de/itm/371898740111? FYI: I have the second version of the lens that is on the right.
Thanks!
Peter

 

[VictorM.]

It's not likely the aperture itself. It's the lubrication under the ring. Scroll down to post 6 in this thread: https://www.rangefinderforum.com/forums/showthread.php?t=50601&highlight=Summitar+lube