I just replaced my 11-25 shimano 105 cassette with with a brand new 12-27. This bike was new about 700 miles ago.

Now, when pedaling in all gear combinations, there is a bumpy or rough feeling that was not there with the old cassette. It's not a clicking or creaking noise, just a feeling in the pedals and a slight low rumbling sound. I've carefully adjusted the derailleur chain tension, limit screws, and b-screw, and I am certain they are not rubbing on the chain.

Is it possible that this rough feeling is because of chain wear? I thought my chain would last a lot longer than 700 miles - I mostly ride on dry, clean roads, but do get caught in the rain every once in a while.

I used a ruler to measure the chain, based on Sheldon Brown's website, and I found about 1/16" of chain stretch. Also, when the chain is on the big chainring, I can pinch the chain in the middle and pull it off the chainring and make a 1/8" gap.

Should I replace this chain? Is there something else that could cause this rough feeling?

Double check your cable tension adjustment. Shift down into the smallest gear, and turn the tension screw in (relieve tension). Then, shift up, the bike shouldn't shift. Slowly turn out the cable tension adjust screw until it shifts up, then go about another 1/2 turn. If that doesn't fix it, we'll take a look at derailleur hanger straightness. And, if you're not quite sure about the B screw, that could be a likely cause as well.

Chain wear isn't just about what weather you ride in, but also how stretched it is.
Sounds a bit like your chain has stretched, while your new cassette isn't worn in, so the links might have a bit of problems falling into the teeth? You could look at the cassette as you pedal your bike on a stand slowly and see what's happening.

I thought it was generally accepted that if you're going to replace the cassette, you're going to need a new chain... YMMV

If the chain is 1/16th inch oversized over a 24 pin interval (one foot) you need a new chain. But I find it hard to believe that you could stretch a chain that much in 700 miles. What kind of chain is it, some are better than others?

Al

I'd just replace the chain as a matter of course when replacing my cassette/freewheel. I'll bet that will fix your problem.

Yes. What you've described is a textbook example of a new cassette and a worn chain not playing well together. I know it's discouraging that you're in that situation after only 700 miles.

It's a Shimano CN-5600.

Replace the chain with the cassette and vice versa.

+1