I measured the stretch of my bike chain with a chain wear indicator. I can get the .75 part to go through but not the 1.0 side so the stretch is probably somewhere between .75 to 1%. Question is when do I replace the chain? The description that came with my chain wear indicator say to change once it reached 1%. Searching the web, some say to change if beyond .75% and if it's beyond 1.0% to change both chain and cassette. So, what the "correct" thing to do??

First of all confirm the stretch by measuring 12" of lightly tensioned chain. The tools tend to read high, so a 12" stretch measurement will be a bit more accurate. On a new chain with 1/2" pitch each pin will line up exactly half an inch apart, with stretch 24 links, which should measure 12" will be slightly longer. Replace the chain when the stretch is more than 1/16" over 12", or 1/2%.

I never replace the cassette when replacing the chain, unless the chain skips. This is likely, but not assured, if the stretch is more than 1/8" over 12". So depending on how stretched your chain is, you might want to go ahead and buy the cassette in case you need it, but hold off changing it unless it's necessary.

Thanks. I think I will go ahead and replace the chain since I have already bought a new one. The current one had seen more than 2500 miles anyway. Looks like I can leave the cassette alone since it's not skipping at this point.

The thing about cassettes is that they usually don't skip with the old chain, unless it's worn outrageously (about 2% stretch). But they often will skip with a new chain. I don't think this will happen to you based on the info you posted, but want you to be aware that not skipping now, doesn't mean your safe.

Good point. Installed the chain last night but have not had a chance yet to test ride. Looking the condition of the cassette, it doesn't appear to be worn so I think I should be fine. I'll find out this weekend one way or another. Having said that, removing the old KMC chain with the missing link was such a pain. I ended breaking the chain with a chain tool. Next time, Park Tool MLP-1! Worth the $15 investment IMHO.

Chain gauges that measure between rollers measure mainly roller wear which is not related to chain elongation and much less important. These gauges will have you replacing chains much more frequently than needed. As already suggested by others, use a good ruler (a steel machinist's rule is good) to measure 12" (center to center over 25 pins) to determine chain elongation. The rule is to replace the chain at 12 and 1/16". I choose to replace my chains at 1/32" of elongation to reduce wear on cassettes and rings. This usually occurs between 2500 and 5000 miles on the chain.

FWIW, I have discussed this issue with one manufacture of chain gauges and they agreed.

I agree with Looigi regarding the shortcomings of chain checkers. In addition the steel rule method averages the wear across 12 full links of the chain not just two adjacent rollers; I generally check a couple of places on the chain as well to get an idea of the uniformity of wear. And, a good steel rule is handy to have around for more than just checking chains.

That's interesting. Maybe that's why on the instruction that comes with the chain wear indicator, it says to replace once it has reached 1% as oppose to 0.75% on almost all other places I have looked.