Gears slipping after a chain change

Typically you'll want to change your chain and cassette together, since they will both be worn, so it's not surprising that you're experiencing some slipping. It's also possible that your existing cassette is bent or damaged in some way rather than just being worn. I had an old XT 9-speed cassette where I somehow bent a few teeth to the right in the middle range, and even though it was barely noticeable, it made those middle cogs totally useless under load.

I assume you also bought a new chain that's the appropriate width for your rear cassette (9, 10, 11-speed, whatever). So if it's adjusted well and you have the right chain, I think it's safe to say that you'd benefit from also replacing the cassette.

Look up the proper chain for the components. XT is not a size - that is the group level. You should size the chain for the number of speeds (gears).

Inspect the cassette. As they wear the teeth get sharper spaces get longer. Visually compare it to a new cassette even if you are looking at a picture.

If your chain snapped then possible something else is damaged. Inspect all the drive components.

A cassette typically last 2-3 chains. If you get a new cassette then change the chain.

A worn chain wears a cassette down faster.
And a worn cassette wears a chain down faster.

A chain is cheaper and easier to change so cheaper to stay with a fresh chain.
There are tools to measure chain stretch.
Replace a stretched chain.

A new chain on a worn cassette will jump - basically both the chain and cassette stretch.

Chain slip, after replacing a chain, usually indicates worn cog(s) and/or chain-wheel. This can be avoided by replacing the chain early, before major wear has taken place. The snapped chain is problematic. The break could have been caused by an extremely worn chain. If the original chain had no master-link, it could be that the pin was not installed exactly as it should have been. This pin could have popped out on a well worn chain. If you still have the broken chain, re-install the chain, replacing the broken link with a quick-link (master-link). Now, does the old chain slip? (be careful during testing) If the old chain does not slip & the new chain slips, then you will need to replace the cassette in order to use the new chain. (one or more chain-wheels may need to be replaced also)

[edit: with a few more years of experience . . ] It is likely if you have new chain and an old rear cassette which is worn then that is the problem. A new cassette will match teeth on new chain.

But the problem could also be with your new chain length or rear derailleur. Happily not expensive just can be a bit tricky to fix.

Three things to check:

1. New chain is the right length? The new chain should have the same number of links as the old chain. Did you line up old and new chain (allowing for wear so the line-up will be asymmetrical!) and put on new chain with the same number of links? I used this recently: http://www.bicyclinglife.com/HowTo/ChangeAChain.htm The old chain will be "stretched" so will not line up exactly with new chain.

2. OFTEN rear derailleur can get a bit twisted or bent. You were riding in the forest and chain snapped. Might you have bumped something? :-) Especially your rear derailleur is exposed to bashing off things when mountain-biking. Check does your rear derailleur cog line up correctly with your rear cogs? Applying some gentle brute force in the right place on the rear derailleur can help. This is very good: http://www.sheldonbrown.com/derailer-adjustment.html

3. Check something in your drive chain. A few years ago I had a bike where I got a notch in my front derailleur which chewed and snapped chains! So check front and back drive-train. Is your back wheel aligned correctly also?

Had the same problem. Just rechecked and had a stiff link where I attached the new chain. Hope that helps...

I replaced an old, barely skipping chain with a brand new one, and it skipped a lot more, right away. I finally figured out that it was too long (by 3 links), but by this point, my two most commonly used sprockets had been worn down by all the slipping.

When I was looking around for this same answer a while back, I found most people talking about worn teeth/sprockets, narrow chains, and minor adjustments. I think a properly sized chain is the best foundation for avoiding this problem, and should precede all the other troubleshooting steps. (Yet again, Daniel R. Hicks has an accurate comment - it should be an answer, so I'm making it one!)

Any chain sizing method can be used, but I used the following, and it worked well. (From https://www.ilovebicycling.com/determine-bike-chain-length/)

Largest cog and largest chainring method

The easiest way to determine bike chain length is the largest cog to largest chainring method.

Once the old chain has been removed, shift the front derailleur to the largest chainring, and the rear to the smallest. Wrap the new chain around the the largest chainring (at the rear), making sure that if the chain has an outer plate, it is routed toward the front chainring. Pass the chain through the front derailleur cage and onto the largest front chainring. Hold chain at the 5 o’clock position. If you are using a masterlink chain, install half the masterlink onto the front end of the chain to account for the extra half link the master link provides. Pull the lower section of the chain snug towards the front chainring, bypassing the derailleur altogether. Find the closest rivet where the two ends could be joined and add 2. This is your cutting point.

If an old chain is jumping in all sprockets , it can only be the chain. (Sprockets dont wear at the same rate. I reckon it will be an incorrect new chain. My new kmc is too wide for my cassette. It does not bed down! Even though both cassette and chain are both nine speed and new. Very frustrating.