I had a flat on my rear wheel. My bicycle is equipped with Shimano Nexus SG-7C18 7-speed coaster brake and in order to change a tube you need to remove both the chain and the internal Nexus 7 hub cassette joint.
That part seemed easy to do, but the trouble came when I tried to reinstall the Shimano Nexus 7 hub cassette joint with cable and lock ring. Even when hub twist shifter was set to speed 1 as advised and had the matching yellow arrows (shifter cassette joint) and dots (lock ring) properly positioned, lock ring wouldn’t lock by turning. It seemed as there wasn’t enough space between the ring and the cassette joint as the lock ring locks in grooves behind the casing.
After I took the bicycle to a mechanic, he told me he had a hard time locking the lock ring, too. His advise was to put shifter in position (speed) 1, put the cassette joint on the hub, use your fingers to open (tension) the cassette joint as wide as possible and this creates enough space for the lock ring to be locked. I hope I won’t have to repeat this procedure as it seems overly complicated. One would expect better from Shimano.
Solution from the discussion below:
I had this problem with my Nexus 7 too, and no amount of fiddling would create enough room to get the lockring to rotate. It should rotate freely by finger, if it doesn’t, don’t force it.
What I discovered is that the driver plate (part 21 on the technical document) was in the wrong way around – it won’t be obvious as it doesn’t stop everything fitting together, and even the installation diagram doesn’t suggest it has to go in a certain way, but if you look closely at it, you can see it has a slightly different profile on each side. I flipped it over, put everything back again and hey-presto! The lockring spun closed first time with no effort.