HELP! My Mamiya RB67 Sekor 65mm lens rear cap is jammed!

[mossbloom]

After today's shoot, I popped the plastic cap back on my 65mm lens, turned the locking ring very gently, and put it back on the case. When I checked it tonight, the locking ring was flat jammed. I've tried a mammoth pair of channel locks and gently tapping it with a screwdriver/hammer, but nothing unfreezes the ring. It did it once before, but I was able to twist it free. It's a great lens with nothing wrong with it, but it's starting to look like curtains for kitty. Any ideas anyone? Al

 

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Is this the cap that has the two tabs or ears on it? I assume it is if the locking ring is turned to hold it in place.
My best and only thought is to make sure you're turning the ring in the correct direction. The only time I've had a similar issue I was not turning the ring in the correct direction. Easy enough to be confused looking at the lens from the back.

If looking at the front of the lens the ring is turned CCW or towards the left and red center dot. If looking from the rear it is turned CW or to the right.

I would give serious thought not to use any channel locks on the ring. I would entertain using a rubber pad for opening a stuck jar lid.

 

[mossbloom]

Thanks. I'm concerned about the right direction, but there's a narrower band on the ring which has to be aligned with the red arrow on the lens before the lens installs, and that says I turn the ring clockwise, looking at the back (camera) end of the lens. And rubber pad's a good idea. I ordered a pair of strap wrenches from McMaster-Carr ($11 ea) and will see if they do anything.

 

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and that says I turn the ring clockwise, looking at the back (camera) end of the lens.

That sounds correct as I've updated my first comment with that same information.