onyourbike

I have just installed a Sram PG830 cassette on my Sora 8 speed bike.
The current chain is a low mileage PC850 and the stretch appears to be less than 1/16.
Since the cassette is on the cheap side and the chain is not overly worn I wonder if it's worth getting a new one?

Barchettaman

Well, 6/7/8 speed chains are cheap, you could swap it out and keep your current one as a spare.

Best of luck.

dsbrantjr

You will only need to replace your chain if it skips with the new cassette. If it works OK you should be fine. As Barchettaman says chains are cheap so it wouldn't be a major expense to replace it to ease your mind.

MudPie

When I put on a new cassette, I always put on a new chain (that's just me). I always like to start "fresh".

But when I replace a chain, I don't always replace the cassette. In general, I can get three chains per cassette.

In your situation, I would install a new chain, and use the current chain to replace the new chain when it wears.

It's good that you measure the chain for elongation - that's your best way to know when to replace.

FBinNY 

There's lots of legitimate debate on this subject, and a number of approaches to getting the most piles for the total chain and sprocket dollar.

1- the most common approach - is replace chains when they wear to a measured 1/2% stretch (1/16" over 12") and replace them. Replacing at 1/2% stretch usually does a decent job of saving the cassette, and most are able to use the same cassette for 3-4 chains.

2- run forever and replace all. Run the chain and sprockets until they give up the ghost and start skipping. Usually this is well past the 1% stretch point, and can be as much as 2-3% stretch. the method can be surprisingly economical, but if carrier too far can also mean replacing the chainrings (or at least one) too.

3- the 2 (or more) chain theory. This combines elements of the other 2 approaches and is the most economical method, especially if you have reusable connectors. use 2-4 chains and rotate them at intervals well short of where they'd reach 1/2% stretch. I rotate 4 chains every 1,000 miles on my commuter. This is a big expense at the front end, but generally allows you to run all the drivetrain components until the chains all reach 1.5-2% stretch. It also has the benefit that chains can be properly washed, dried and oiled at your convenience while out of the rotation.

The worst method is to run a chain to about 1% stretch, and then replace it. By now the cassette is beyond saving, so you might as well leave them together and adopt the 2nd approach.

Which method makes sense for you depends on how much you ride, how long you keep your bikes, and the relative cost of chains, cassettes and chainrings. years ago, the use until it dies made sense since freewheels were fairly cheap, but with today's expensive cassettes methods 1&3 make more sense.

FastJake

At less than 1/16" stretch I think you'd be fine leaving the chain on there. But me personally I would put the old cassette and chain in a bag together and use a new chain with your new cassette.

Did you put on a cassette with a larger first cog? If so, you should put on a new chain regardless as your original chain is probably not long enough. To determine chain length: https://sheldonbrown.com/derailer-adjustment.html#chain

Lawrence08648

I replace my cassette every other chain change, chain every 4,000-5,000 miles, cassette every other.

pat5319

Run it if it doesn't skip and get a chain measuring tool if you don't have one, it eliminates guesswork and saves time. I'm sure mine has saved me money and one of my fave tools.

Chain wear depends on weather, terrain, power applied, seasons, how clean you keep your bike, proper lubrcation.... the tool will TELL you when to change.
A team mechanic, i used to ride with, told me that Sean Yates had to change his chain every 500 miles.

onyourbike

Yes, I replaced a 12-25 with an 11-28. I have 39/52 up front and have shifted to the big/big combo, seems OK.

Looigi

FWIW, I was a die-hard adherent to this approach with all my 9 speeds, but in my most recent experience I religiously measured my chain, replaced it just shy of 1/16" elongation, and got skipping on the 4th largest cog on a 10 speed SRAM Red cassette, necessitating replacement of the cassette or going back to running the worn chain. I don't think it has anything to do with 9 vs 10...maybe I just wound up using the 4th cog an awful lot or something... In any case, I'm now leaning toward simply replacing the chain sooner before it reaches 1/16" of elongation. Dunno how soon, 1/32, 3/64??? I prefer to use chain elongation as the determining factor rather than chain measuring devices which include 2x the roller wear, which up to now I didn't think mattered, but now with the teeth on the cogs being so short, I'm not so sure.