So I finally measured my 10 speed chain, not due to any issues, just because I see it recommended so often.

Well, it's 3/8ths stretched. It's about 3 years old, around 4000 miles on it. FSA double crank, and Ultegra 6600 rear cassette. I dont have any skips, nor do i see any strange wear issues. No shifting issues that I am noticing.

So, it appears my chain is well past due, but things I read say that the cassette may be damaged at this state of the chain, replace it also. Some people say just change the chain, and see if it skips. Some people say ride it until theres a problem.

What might you all say?

Thanks.

Is that 3/8" elongation over 24 pins or over the entire length of the chain? If it's over a nominal 12" (24 pins) your chain, cogs and chainrings are ruined. If it's over the entire chain (typically ~112 pins) you are just over the normal replacement value of 1/16" in 12" and your chainrings should be fine and the cogs may or may not be.

3/8" in 12! You will be lucky if the chainrings aren't toast. You will need a chain and cassette ast the minimum.

https://draco.nac.uci.edu/rbfaq/FAQ/8d.2.html

I have to assume the OP is measuring the entire chain's length. I don't think King Cong could "stretch" a chain 3/8" per 12" in only 4000 miles.

You can let everything wear out and replace all, (chain, chain-rings, cassette), this can add up big and quick
You can measure your chain regularly with a ruler and hopefully change it before your rings and cassette is worn.
You could buy a chain measuring tool, (Park, Rohloff etc.,) eliminate judgment calls and save time, trouble, effort, and guesswork and be sure to change the change the chain before much wear occurred. (Wear can’t be avoided entirely)
Avoid using a “favorite” gear. I’ve seen way too many cassettes needing to be replaced because one cog was worn out. Change gears as you change speed, save your cassette and your knees etc.

Just be glad your not Sean Yates, in his peak years he HAD to change his chain every 500 miles, can probably get by with 600 or 700 now.

I hate to have to admit this, but it's stretched 3/16ths (over 12 inches) NOT 3/8ths. Sorry about that.

Ugh. So I'm guessing now would be a good time to change out that chain before major damage sets in? Likely to be fine?

Thanks.

It's a pretty safe bet your cassette is now a throw-away too. Your chainrings may or may not need to be replaced as chainrings are pretty wear tolerant. Replace the chain and cassette immediately and try the current chaings. If they shift and run properly, be thankful.

Even 3/16" is probably past the point of major wear. Most replace after 1/16", some go to 2/16"(1/8"). You'll probably need to replace the cassette as well

Thanks.

What eventually happens? Gear skips, chain break, etc?

Possibly. Could also have a chain derail. You could be thrown off the saddle and smash your genitals on the top tube. Or be thrown over the bar onto your face. Or into the path of a bus. Not trying to be alarmist, but big bad things can happen from little things. You know, for the want of a nail and all that.

+1 You should at minimum replace the chain so the worn chain doesn't cause further wers damage to your dirvetrian. While chain skipping and bad shifting can start to happen your biggest worry with a worn chain is that it will derail at a bad time potentially causing a serious crash.

I went ahead and ordered a new cassette and chain. Not worth worrying about it. Thanks for all the feedback.

...and also had to order a new 39T ring. Old one was complaining loudly with the new chain. The new ring is on and is silent.