Semi-hypothetical question:
When I was a kid, I'd run chains and sprockets until the teeth got hook-shaped before I replaced them. The biggest bike I did this with was a Yamaha XS650. I also changed chains without changing sprockets because the chains I was using were dirt-cheap. While I'd never do that now I also never had a problem with it.
So here's my question. If chains and sprockets must be changed together, why not shorten an existing chain when it runs out of adjustment? The reason I said "semi-hypothetical" is because I used to do this on a Triumph 955 I had. I'd only get a few hundred extra miles out of it, but it was enough to get me to the next payday.
I'm interested in the thoughts of people with ACTUAL experience, not ideology or "my cousin's brother's boyfriend's dentist says..."

 

Sprockets dont HAVE to be replaced with every chain replacement. If they look like they are in decent shape, they can be used again.

Now as for "chain stretch" and shortening the chain.... The metal doesn't stretch and make the chain longer. Its the increasing clearance inside the rollers that make it seem longer. A little play times all 108 or so links really adds up. Simply making the chain shorter doesn't help the wear at all. Its still going to wear and eventually break and destroy the engine case and maybe your leg.

 

I didn't think that the metal was stretching. The holes in the plates get bigger and the pins going through them get smaller. This is why lateral bending on a new chain us far less than on an old one.
As for not replacing sprockets and chains together, you're the first person iI'v talked to in years that said it was okay. I'd be inclined to agree up to the point that the (steel) back sprocket gets far less wear than the front, making it last longer.
But the original question was - since the common wisdom is that chains and sprockets wear together, effectively ruining all of it, why not shorten the chain and get a few more miles?
In 30+ years of riding I've dropped two chains and it was because the master link clip failed.

 

I've got more than a few miles on F2's, so this is real world experience. Do with it what you want.
I usually replace the front sprocket along with the chain. The fronts are smaller so they get a lot more wear than the rear.
They turn about 3 times faster than the rear. I usually change the rear every 2-3 chains if steel, every chain if the rear is aluminum.
I've done this for a long time, but you need to visually inspect everything at each change and make sure it still looks good.
And it depends on conditions, how well you lube and adjust your chain.

When a chain stretches, the spacing between the links gets longer. Sprockets don't stretch. So with a worn out chain, what you end up getting is all the pressure of the chain being taken up by a single link, not being spread out over the normal handful of links. This causes 2 things to happen, you wear your rear faster and end up having to replace it soon. That will cause you to lose any cost savings by trying to run the chain longer. When a chain is done, you replace it, that simple.

2nd, all that force on a single link can cause the chain to snap. I've seen it more than once. One time at about 140mph. It wasn't pretty. When they break, they frequently go through the back of the engine case. Then all the oil blows out under your rear wheel. You crash and end up going to the hospital or a funeral home. Again, in either case, you really aren't saving anything and most likely you aren't too unhappy either.

 

Oh, one more thing. F'k master links. Rivet it or stay home.
There is no good reason to a clip, ever, period, full stop.

Not that I have a real strong opinion on this subject.

 

Shortening it doesn't correct the part that's wearing out. The actual rollers and steel teeth don't wear quickly if they are lubed and maintained. If the pins didn't wear, chains and sprocket could last the life of the bike.

Master links are usually the first to go because they are not put in at the factory...

 

As phil mentioned, the front sprocket will wear faster than a rear. As well as checking the teeth, the inside takes a ton of abuse from the output shaft. It must be tight on the splines or it's done.

The cush rubbers in the rear reduce the blow to the rear sprocket. If you ever feel like replacing those, you will notice a huge difference and see just how bad they can get when they're old.

With all that said, replace the slider with every chain. Your swingarm will thank you.

 

Actually the clip kind will pass race tech with safety wire (at least in WERA), although I prefer the rivet type.

Speaking of racing, part of the reason I brought this up is that the experience I gained there changed a lot of what I thought mattered with regard to working on bikes and I like hearing what other real-world experience is out there.

For instance, I never would have dreamed that warped brake rotors could be straightened trackside, or that top racers don't bother with electronic wheel balancing, they do the same static balancing I do in my garage.

The uneven stress on sprocket teeth from a worn chain is a valid point, but that's something that happens in degrees over time, not all of a sudden. Obviously a new chain is better, but when does it really start to matter?

 

I've raced cra, ccs and wera. There's a lot of stuff that will pass tech. That's the low bar. I've got a higher standard when it comes to my safety because I've see it go wrong many times. If my chain is questionable, it gets changed and riveted before I get to the track. I've seen too many people at the track rush something, do it wrong or incomplete, and then things go from bad to worse. And yes, racing will definitely teach you what matters and what doesn't.

Yup, you can definitely straighten rotors if they aren't too warped. All my tires are hand balanced, either by me or at the track. Test good to 185mph.

Yes, it does happen over time. Chains like everything else are build with a safety factor. But generally 1% stretch is considered the limit. That's where the mark on the swing arm is set. Once you go past that, you don't have as much safety factor. Hit a pothole and send a shock load through the a worn chain, it could snap. Of course it all depends on your setup and riding style. I don't worry much about a 530 on my worn out F2. But the 520 on my gsxr1k racebike gets changed religiously before the wear limit.

 

My F2 is my fast bike 😄

​​​​​The little bit of racing I did was all in WERA vintage and AHRMA. I certainly wouldn't go to the track with an iffy chain and now that I have the tool for the rivet master links I'd go that route for a sport bike.

For safe street riding and certainly for my old Brit bikes I don't think the clip type is really a problem. For my rat chopper anything goes.

Speaking of passing tech, I once did a track day on the FZR600 I was racing in WERA and the tech guy raced in CCS. He had a bee in his bonnet about WERA's tech being too strict (I've never raced in CCS but this is what he thought). He saw the WERA sticker on my fairing and said "oh Christ you probably have everything wired" and passed me without looking at anything. I went back to my pit and rechecked everything myself.