My bike (Shimano 105 components) has right at 4,000 miles and the current chain about 2,000 of that. I measured the chain and 24 links were 12-1/16". I guess that theoretically it is ready for a chain and rear cassette replacement but it still functions well. Since I'm supposedly already in it for a new cassette, what's the downside in just continuing to ride it until it starts to screw up?

Unless there has been some sort of unusual wear, I doubt that you need a new cassette. I have over 11,000 miles on the cassette on the Fuji and a little over 7,000 miles on the cassette of the Cannondale. Both are the original cassettes. Get a new chain to keep from wearing the cassette before its time.

For what it's worth, I've been using KMC chains as replacements, and I like them. They are quiet, and they have the "missing link", which makes it easy to remove the chain as needed for cleaning.

Relatively new at this, I just look for a chain for a 105-10 speed and the length will be correct? Need special tools, maybe a chain breaker? Current chain was installed by LBS, it is SRAM and has a master type link. Tried to use needle nose for removing master link so I could clean the chain a while back it but didn't work. Got lots of motorcycle mechanical experience but very little bicycle.

I have a Park chain gauge and I check my chain length every month. Boreas is on its 3rd chain in 6,000 miles and it is showing .5% wear so still plenty of life in it. Cassette is still in pristine condition. The TCR has .75% wear and I already have the Ultegra chain in the spares box for when it gets to 1% wear. Cassette is still good so no worry there and this is after about 2,000 miles

But last week I checked a Cheap Chain on a Cheap bike. Just on wear limit and no problem with the cassette. That was after only 500 miles

Problem is that if you have a worn Chain and a worn cassette- and only change the chain- then you can have skipping on the gears. The new chain does not like running over a worn cassette. So visually check the cassette when chain time is up. If in doubt keep a spare cassette in the spares box.

And when you buy a chain- it will need cutting so get a chain breaker and either a Quick link for the chain or a new Rivet if it doesn't come with the chain.

To get the right length- put the chain on the big ring on the Crank and big ring on the cassette. No need to run through the derailler. Break the chain to "Over" length and never cut it too short.

Personally I always use Shimano Ultegra chains as I have never had a problem with them but "Apparantly" Some have noticed that the newer Cassettes and Chainrings need the newer chains- This may only be on the top end stuff but ask the LBS and they can always sort the right chain for your series of Groupset.

All of the replacement chains I have purchased (and one was a 105 chain) have extra links. You will need a chain breaker to adjust the length. I bought a little pliers-looking tool from Park Tool (MLP-1) to squeeze the missing link together a little to pop it off. Works great.

I got Zinn and the Art of Road Bike Maintenance and followed the instructions for installation of a new chain. There may be tutorials and/or YouTube videos on that topic as well.

^^this. Plus, if you buy a shimano 105 or ultegra chain they come with instructions that are easy to follow.

If the chain is only 1/16th too long it is unlikely you'll need a new cassette. Typically, my cassettes will go through five or six chains, and I generally change them at the 1/16 point.
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I agree with doctor J, You shouldn't need a new cassette yet.

Assuming your old chain was cut to proper length, lay it out on a flat surface next to the new chain and use it as a gauge for cutting the new one.

I replace my chain every 2000-2500 miles. It saves unnecessary wear on the cassette so it doesn't need replacing as often.
Chains are cheap.

Thanks for the help guys but one more thing. I see that SRAM chains come with a master link but there's no mention of that with the Shimano chains. Buy that separate?

Edit: never mind, found this https://www.bicyclinglife.com/HowTo/ChangeAChain.htm

Nothing wrong with Sram chains and they do have the master link. Shimano use the "New" rivet system.

If I were you- go with the Sram chain but get the right Type for the speed of your cassette and Buy a high enough quality.

Reason for using the Master link as a Novice chain fitter is that getting the Rivet in correctly can be awkward. It can be done too tight and give you a tight link that is awkward (But not impossible) to correct.

You don't have a master link with Shimano, you use the chain tool to push a rivet most of the way out, break the chain and then reinsert the rivet. The chains come with spare rivets to make the connection after yu've broken the chain. It sounds complicated but isn't, and as Stapfam says, if you get a tight link through inexperience it is possible to correct it, again using the chain tool.

I have tried Shimano 105, SRAM and Shimano Ultegra 10 speed chains. I found a good deal on a Dura Ace 7800 10 sp chain; so it is my current chain. I typically only get about 1200 mi on a 10 sp chain. I haven't had any cassette problems. I regularly clean and lube my chains. I am using the purple lube. I attained much greater life on older 8 speed chains.

I usually get at least three chains per cassette change. I tend to ride a chain too long (mainly because I keep forgetting to check the mileage) so I frequently get 2000 or 2500 miles per chain.

You can also buy the quick link from either KMC or SRAM and use it with a Shimano chain, it depends on your chain cleaning procedure, if you like taking the chain off and boiling it in chain cleaner and wax before putting it back on, then go with the quick link, if you don't care to remove chains to clean them, then go with the replacement pin from Shimano. I use only KMC chains which come with the quick link, so it doesn't really matter much. With 10 speed, you don't really need the quick link, the chains wear out much quicker so, you don't give it a good cleaning, you just replace it.

It's funny a 6 speed chain costs $10, and is good for 10,000 miles, an 8 speed chain costs $20 and is good for 5,000 miles, a 10 speed chain costs $40 and you consider yourself lucky if it lasts 2,500 miles, why is this?

A SRAM chain should work well on your bike. I use SRAM chains and I have Shimano 9 speed components. The cassette wear will accelerate if you are running a worn chain. It is best to get the little Park tool chain wear gauge. It is really quick and easy to use. I check the chain wear everytime I clean the drive train. I usually can replace a fair number of chains without changing the cassette. Since chains cost far less than cassettes, that is a good thing.

This might help with removal of the SRAM power link:

https://www.sheldonbrown.com/brandt/power-links.html

This really helps.

https://www.amazon.com/Park-Tool-MLP-.../dp/B000AA30BG

I find that sometimes I can easily open the link and other times I just can't seem to get it right. The tool works every time.

That must be why I simply could not open the master link with a standard pair of needle nose. I just ordered a new SRAM chain, a Park chain pin tool and special pliers for removal of master links.

That is a good looking bike!