I have a 2013 Trek 7.1FX. My chain wear tool dropped right in so I thought it was time to replace the chain. The existing chain is marked "KMC Z Narrow". I thought I did my research and ordered a KMC 51 chain. It is also marked "KMC Z Narrow" on the chain itself.

The master link looks fully engaged and it rotates freely at both ends after I installed it. On my entire 10 mile ride yesterday the chain "skipped" or it seemed to miss a tooth every now and then in the rear. (7 speed). It never did this before with the old chain. The chain never jumped off but it was extremely annoying and something just isn't right.

I guess I can take the master link off of the new chain and use the old chain until I figure this out but that's not where I need to be. Did I buy the wrong chain or what suggestions do you have?

Front: Suntour XCC, 48/38/28
Rear: Was...MF-TZ31 7 speed 14/34 and replaced last year with DNP Epoch Freewheel 7spd 11-32. No issues with either Freewheel with the old chain.


Thanks.

If your chain was that worn, it may have worn the teeth on the freewheel as well. They are cheap enough- just keep the old chain on until you get the replacement freewheel.

At first glance, this seems to be a classic "new chain skips on old sprockets" issue. This is extremely common and very likely if the prior chain were very worn. Stretched chains create a slightly hooked wear pattern on sprockets which cause new chains skip. If it's very minor (doesn't skip all the time) it'll self resolve as the chain wears away the offending areas, but serious or chronic skipping will wear you out before it wears itself out.

As I said, that a quick diagnosis based on the obvious clues, but doesn't rule out things like a trim adjustment which is an easy fix. Do the regular checks, and see if you can live with i a while, but is sounds like you need a new cassette.

Thanks. I was afraid it might be the freewheel. I'll run it for a while and see if it improves or how long I can stand it.

I guess the lesson learned is I should have put a new chain on when I installed this new freewheel last year.

Other than wear , mentioned above , we count how many cogs are stacked up on the back wheel cluster...

Aka 'speeds' , that determines width of chain. 5,6,7,8 are using the same width.. KMC chains as a brand are good,

yes, it's time to buy a new 7 speed freewheel .. to go with the new chain

Or, wear the new chain out prematurely..............

Don't run it too long or the new chain will quickly wear in to the worn freewheel and you will be back to the same problem with the new freewheel.

EDIT: Wanderer beat me to it.

Freewheel?