I have an 8 speed cassette on a wheel that's used mostly on my Diamondback Outlook which has a 7sp RD.
This wheel and cassette came with my 24-speed Sorrento when I bought it used. You can see the cluster has some rust on it but it's worked well for the past few months. I plan on keeping the wheelsets swappable, so I will probably get another 8 speed cassette unless you think that's part of my problem.
I have dialed the rear derailleur in so that it uses the outermost gears, never touching the granny gear on the cluster. A few days ago, I started to get slipping when in the 11t (smallest, highest) gear but only when I'm hammering.
Do you think it needs replaced? I'm assuming it's the original cassette that came with the Sorrento which would make it about 8 years old with unknown mileage. I have put about 400 miles on this cluster since I got it in September. I'm splitting my commute between two bikes depending on the weather and this wheelset mostly stays on my "Fair weather" bike.
Anyhow, on with the questions:
1) Does the 11t sprocket look worn out to you? I don't think it looks quite right.
2) The spacing of gears IS the same between an 8sp and a 7sp cassete, right? I didn't think there was a problem running this setup. I like being able to use either wheelset on either bike in a pinch.
3) The small sprocket is the lock washer on this cassette. Even though the rest of the cassette is ugly, I haven't had any problems with other gears. Could I just buy a new "11 tooth cassette lock washer" or would that be unwise?
Shimano 7 speed spacing = 5mm c/c between sprockets, 8 speed = 4.8mm, so over the 7 sprockets you're using you are only off a total of .2mm*6=1.2mm, which if you center this it is only 0.6mm max on each of the outside sprockets, less as you work in to the center. You may just need to loosen your cable a touch, and/or a slight adjustment to your limit screw.
Sorry, do you have a "7-speed" rear deraillieur (marketing term) or 7 speed shifters? I'm just not sure why you have put the limit screw on for only 7 of your 8 speeds. If you did so because you thought that the derailleur was speed sensitive then you should be able to use them all by readjusting out your limit screws. its possibly as well that you overstretched the cable by having the last cog limitted out by occasionally trying to push into it anyway.
otoh, if you limited it out because you have a 7 speed shifter then maybe you could change your operating range from 1-7 into 2-8 It would give you a slightly lower range, but maybe you could learn to spin more that way you just wouldnt be using the troublesime cog.
clean drivechains run smoother. it looks a bit grimy.
its not a great focus shot, but it does look like a fairly worn chain and cluster, so it might be at the point of (a) replacement anyway or (b) more meticulous maintenance.
ditto on the spacing 7 vs 8
ring a few bike shops re the 11 tooth cog. Ive been given one by friendly lbs' before. you neve know whats hanging out in a box out the back no matter how long ago it was discontinued.
The shifter for the RD is a 7-speed twist-grip and the RD says "Shimano SIS 7" leading me to believe the RD is made for 7 speeds.
On my Sorrento with an 8-speed rear trigger shifter, the RD says "Shimano 8SIS".
The long story is: The hybrid came with a freewheel-style rear wheel. Before I even rode it 5 miles, I bought a freehub wheel and new cassette, put the rear tire on that wheel, and swapped the knobby wheel set onto my mountain bike. The indexing on my outlook is actually set up for the original freewheel cluster (no clue if 7-speed freewheels and 7-speed shimano cassettes are differently spaced).
I just got in from a commute when I took that photo. It's usually a bit cleaner than that, but with the slush and crap in Kansas City, the grime is just part of the commute. I clean it every few days.
When I initially adjusted the RD, I did it in whatever gear it was in at the time. It's very possible that I just need to make sure I'm in 4th gear on the rear cluster and adjust it that way. I had no clue that there was a spacing difference.
In that picture, it looks very much like you have a lockring outside the top cog, hyperglide style. The top cog does appear to be very worn. I was not aware that threaded top cogs could come as a 11 tooth, it would seem to be too small to be able to carry an internal thread.
In any case, a new cassette and chain will most likely fix your slipping issues.
As to rear derailleurs, one thing Shimano has been good about is that they have kept the pull ratio of their derailleurs designed for indexed shifting constant throughout (with one exception for old Dura-Ace) so that any RD will work for any shifter, you just need to match the number of speeds on the shifter to the number of rear cogs, and there are some number of cogs combinations that are pretty close in spacing so that you can get away with mixing, 7 and 8 speed is one.
since you have a spd shifter on the 8 at the back you wre right to use the limit screws to block one out. As i said before, can you just block out the one thats slipping? you'll have an overall lower range, but in general, spinning is better than mashing anyway.
you could try and get a new 11tooth cog, but i've changed my mind. it'll probably fail to mesh with the chain and just slip more...
That's got to be a HG cassette - I don't remember if the smallest available DA cog on Uniglide was an 11 or 12, but you don't have a DA cassette or freehub. The lockring is separate from the cog itself. The 11 does look kind of worn, though not too bad. I would see if you can replace just the 11 - replacing the whole cassette would be a waste of money. On the other hand, you could get a 7-speed cassette and a 4.5mm spacer to put behind it. Running 8 cogs when you can only use 7 is a little bit silly. It it's working for you, though, just get a new 11, no sense in letting your money burn a hole in your pocket. That cassette doesn't look worn enough to justify replacing.
Before replacing that cog, I would check your chain for stretch. You could just be making things worse if you get a new cog without replacing a worn-out chain.
You might want to think about think about changing your riding habits a bit to spend less time in the teeny-tiny cogs - shift to the big ring more often, or try to spin lower gears faster. But that's if you really care. You're just getting to work and back on this bike, right? Do what you want, don't spend too much money, get to work and have some fun!