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I snapped a chain today, on the errand bike near work (in traffic, but luckily not uphill). That's 8 speed, and I have a lightly used 8-speed chain (Shimano). I don't have any 8-speed quicklinks, and the chain is the sort that came with a special pin. I don't want to drive out a normal pin, and drive it back in - that's let me down before.

But I do have plenty of 9-speed quicklinks (some at least are KMC). Testing off the bike, the 8-speed chain and 9-speed link work together.

But am I missing something? Is this just a temporary fix to get me to work tomorrow, that needs refixing soon?

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    For a "temporary fix" I would say fine. That "temporary" though should probably be no more than to get you to the LBS where you can get the correct parts maybe
    – Hursey
    Commented Jul 26, 2023 at 20:47
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    @Hursey funnily enough I can actually get there today, so I can see if I can get an 8 speed link. Otherwise maybe I'll order one. It's not a long distance bike, but beats running quite hard across the city centre to catch my train
    – Chris H
    Commented Jul 27, 2023 at 5:41
  • Why did the chain snap? Was it severely worn? You're not using it anymore, right? You now have two 8 speed chains, the one that broke, and a spare one? None of which have a matching quick link? Do I understand the situation correctly?
    – Robert
    Commented Jul 27, 2023 at 22:35
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    @Robert this was a recently-acquired old bike. I'd put a few tens of km on it, testing it and upgrading a few things. The chain wasn't worn according to measuring 10 links with a ruler. I suspect that a standing start broke a link, but it actually failed when I changed up riding in traffic. The chain came off and ended up under a taxi as I coasted to a stop. It wasn't there the next day. The replacement was one I'd fitted to a previous bike a couple of weeks before a weld snapped on its chainstay. That had served me well, but I saved the most of the drivetrain, still held together by the chain
    – Chris H
    Commented Jul 28, 2023 at 5:43

4 Answers 4

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9 speed should be thinner than 8 speed, so I imagine that if it clicks closed you're good to go.

Nominally 6/7/8 speed chain is 7.1mm wide, and 9 speed chain is 6.7mm.
Is it possible your chain is already 9 speed?

I imagine that a difference of 0.4mm should be enough to stop a 9 speed master link from closing successfully.

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    I had my last 10 speed quicklink go a little sloppy, so I used superglue as a kind of threadlocker to stop it popping apart. Worked for ~3 months when the chain was worn, and I remembered after it was very hard to undo the master link.
    – Criggie ♦
    Commented Jul 27, 2023 at 2:14
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    I can't see the chain being 9 speed. I'd just fitted it when my old 8-speed hybrid broke at a weld. I kept the whole drivetrain in one piece. And the quick links are marked 9S. But by those numbers you're right, 0.4mm means it shouldn't do up, and it did without a fight. It was a bit stiff getting it off again, but not unreasonably - those links always take some undoing
    – Chris H
    Commented Jul 27, 2023 at 5:37
  • @ChrisH I think you're good to go for a long time, at least until that chain wears out.
    – Criggie ♦
    Commented Jul 27, 2023 at 10:48
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    It feels fine, and at 30km/week that chain will last a while. Of course a fair bit of the chain thickness difference is in the outer plates, and that wouldn't stop the link going together.
    – Chris H
    Commented Jul 27, 2023 at 11:35
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I was curious as to why it fitted, so I took some measurements. These were in the middle of the plates - the nominal width is at the rivets so would be expected to be larger. This is a KMC 9X 9-speed chain, and a Shimano 8 speed, model unknown (on a different bike to the one I was fixing, but that's also a Shimano chain).

These are to the nearest 0.1mm. My calipers do 0.02mm, but it's hard to view the vernier scale square on enough to trust, when measuring a chain on a bike

Measurement 9 speed 8 speed
Outside of Outer Plates 6.4 7.0
Inside of Outer Plates 4.5 4.9
Outside of Inner Plates 4.1 4.4
Inside of Inner Plates 2.5 2.7

So the outer plates are a little thinner on the 9-speed, unsurprisingly. Crucially, the inside of the outer plate of a KMC 9-speed is 0.1mm bigger than the outside of the inner plate of a Shimano 8-speed. So clearance is less than normal, but the numbers say it can fit together. Of course non-flat plates would skew the numbers but this was where I could measure without dismantling anything

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  • If the link closes securely into place, then the only issues I can think of is a slower shift once in a while, and a little bit of noise when in some cross chained gear combinations, as the link is stiffer laterally now with those tighter gaps.
    – Robert
    Commented Jul 27, 2023 at 22:11
  • If the cross chaining is extreme maybe the link could break, but I doubt it. Noise and chain skipping are more likely, forcing you to avoid that gear combination.
    – Robert
    Commented Jul 27, 2023 at 22:16
  • Eventually the link will wear down a little bit, you will see round markings around the pins on the inside surface of its plates. Then it will be less stiff laterally, but perhaps not as strong. It's an interesting experiment. Ride it for a while, and tell us how it goes. :)
    – Robert
    Commented Jul 27, 2023 at 22:26
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I'm amazed that you were able to get a 9 speed quick link closed on an 8 speed chain. I once bought a 9 speed, KMC, "missing link" to join a Shimano 8 speed chain. I messed with it for an insufferably long time on a couple different occasions. Couldn't get both of the pins to get into the outer plate holes at the same time so I could then snap them home.

Finally, I sheepishly returned to the LBS and learned that I had purchased a 9 speed quick link for my 8 speed chain. To address your situation, if you've got your 9 speed quick link closed on the 8 speed chain (BOTH pins engaged in their home/closed position) that should be okay. Getting it to unlock may be quite a feat as it must be incredibly tight.

Regarding your comment, "I don't want to drive out a normal pin, and drive it back in - that's let me down before." A Shimano 8, 9 or 10 speed chain requires (according to Shimano) a dedicated, special link pin to rejoin the chain when a regular pin has been driven out. You most definitely will soon have a broken chain if you try and reuse the regular pin you've driven out. A chain joined that way will come apart there in short order. The special chain connecting pin specific to that speed chain must be used. If these are installed correctly, the chain will be very robust. Also of note here, despite Shimano's warnings against the use of third party quick links to join their 8, 9 & 10 speed chains, there is abundant anecdotal evidence that these work just fine and the practice is very common in the wild. KMC's missing link and SRAM's Powerlink are the two most used quick link products to accomplish this.

I suggest two things: first, examine the current connection of the 9 speed quick link (QL) on your 8 speed chain. Make sure the outer aspects of the pinsa are, in fact, fully engaged in the locked portion of the outer plates of the QL. Wouldn't surprise me if one side wasn't truly engaged and that's why it even fits at all. Second, get the appropriate speed of QL for your chain. Inexpensive peace of mind. Or, insert the appropriate chain connecting pin. In any case, get to a more permanent--and safe, and reliable--solution as soon as possible.

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  • When I tested it at home, I bent my quicklink pliers getting it open again. But they were flimsy junk (chain bike shop own brand) and I'd bent them before using a KMC link on the chain it came with.
    – Chris H
    Commented Jul 28, 2023 at 5:45
  • Good point about Shimano's dedicated replacement pins (I tend to forget that nuance). That annoyance alone has kept me away from Shimano chains when possible. The Shimano chains are still good chains; it's just that annoying "special pin" thing.
    – Ted Hohl
    Commented Jul 28, 2023 at 15:20
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I would guess that it would be ok at least for the short term, although chains are so inexpensive that I would suggest getting the properly matching set as soon as is convenient. It may be that the width of you 8 speed chain and link is negligible? Mixing and matching is usually discouraged as it may produce strange shifts that only occur as a certain part of the chain passes by. On one hand, a quick link that is wider that the average link of the chain might be very problematic as it would occasionally want down shift on the rear without provocation. On the other hand, a quick link that is too narrow might bind and would result in the chain "skipping" under load as the link passes over the rear cog.

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  • I wonder if it might be stiff in practice. It certainly took a lot of force to undo it after testing
    – Chris H
    Commented Jul 27, 2023 at 5:31

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