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I broke the master link in my chain a while ago and my LSB told me it was because I wasn't cleaning/lubricating my chain often enough. The chain had experienced more wear than it should have as a result. I asked about replacing the chain and was told that you typically don't want to replace individual parts of the drive-train, that mixing and matching new chain and old chain-rings is a bad idea. Is there any truth to this? Why would a new chain on an old (we're talking less than 4 months) be a bad thing?

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2 Answers

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  1. Breaking a master link, as already said, is not so common, and is usually not related to the chain wear. It can be related to loss of lubrication, for example when the chain RUSTS and gets some hard links that cause the chain to get entangled with the derailer while pedaling, for example. But only because it is worn, supposing it is clean and lubed, I'd say no.
  2. Installing a new chain with an used cassette might be divided in two kinds of situations:
    1. You install and the chain doesn't skip. This is the ideal situation, meaning you replaced the chain at the right time, and your cassette will last much longer (note: this is the recommended thing to do - use lots of chains with the same cassette, replacing them often as soon as they get some wearing);
    2. You install the chain AND IT SKIPS over the cogs when you pedal. That means you waited too much to replace the chain. It might be very difficult or almost impossible to use the new chain and make the cassete wear "back to normal". In this case, it MIGH BE a better idea to keep riding with that chain for a bit more and replace it together with the cassette later (waiting too much can be dangerous, since the chainrings might get damaged in the process). Even when the chain skips, though, if it skips only while pedaling hard, and/or only on specific gears, it is usually possible to ride slowly in the beginning, and the skipping starts to disappear as the cogs wear, and everything might come back to normal.

So, the shop guy may or may not have told you the truth, depending on how much your cassette is worn. This is not an easy thing to do if he didn't measure the chain with a gauge, so it is possible that is obvously worn, or he has a very good eye, or he actually wants to sell you the whole kit. Besides, the need to change everything together is surely NOT true, since the expected is to change the chain often, so as to preserve the gears, which can last MUCH longer.

I would do the following: buy a new chain and try. If it works, it works. If it skips, I put back the old chain and keep the new one at home. When the time arrives, I change the cassette and then install the new chain.

(Finally, some crafty people are able to remanufacture the shape of the cassette teeth by disassembling the cassette and correcting the teeth shape with a round file, one by one. I did it successfully a lot of times, but it takes some skill and a lot of time and hand labor, sort of a masochistic hobby, but anyway it works and can save you some money or at least the trip to the shop.)

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(A Dremel tool will make the cog dentistry go much faster.) –  Daniel R Hicks May 20 '12 at 19:00
    
@DanielRHicks I had tried this with the Dremel, and didn't like it. It tends to climb over the tooth, and requires an uncomfortable force from the wrist. As with many other things I planned to do with the Dremel when I bought it, this is one more job where manual tools end up being easier, not to say the round file gives more "bite" to the teeth, so that they grab the chain much better. –  heltonbiker May 20 '12 at 19:49
    
(side note: Dremel is the brand name of a very nice hi-speed rotary tool. It is ideal, for example, to rectify the shape of cable housings after cutting them with a common plier, or to remove eventual sharp edges from the inner side of rims). –  heltonbiker May 20 '12 at 19:52
    
Sound advice. Will pick up a new chain next week, thanks. –  meagar May 20 '12 at 20:41
    
Yeah, a Dremel is definitely hard to control. You need the sprocket to be firmly fixed (not twisting), and you need to arrange a way to brace the Dremel, then grind very carefully. –  Daniel R Hicks May 21 '12 at 2:02
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How many miles do you have on the bike? How worn was the chain, when the bike shop guy used a gauge on it (which he should have done)? (Note that usually shifting problems and chain skipping are the first signs of a badly worn chain. A failed master link is not usually the first sign of wear.)

Generally a chain should be replaced about every 2000 miles (when chain stretch measures about 0.75%, and certainly no more than 1%). If you wait longer than that the sprockets "take a set" to the worn chain and a new chain will not fit as well. But if you replace the chain before it's that badly worn then you can get 5000 miles or better from a rear cluster and 10K miles from your front rings.

The "replace everything together" "rule" is stated for two reasons: 1) It makes the bike shops money, and 2) most people don't replace their chains until they're too worn.

But even then, the rear cluster is usually the seriously worn piece and the front rings are often not bad enough to need replacing. And, in any event, a competent bike shop should have a gauge for measuring sprocket wear, vs just using "the rule".

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The bike is about 4 months old, and has been getting between 150 and 200km/week of usage. He did use a gauge, and told me the chain had much more wear than it should for a bike only ~4 months old. I'm trying to drag this out of memory, but the thing read "twice" what it should, give or take. They chalked it up to insufficient cleaning/oiling of the chain. When I asked if the chain should just be replaced, he told me the above about not mixing and matching old/new components in the drive train. –  meagar May 20 '12 at 20:39
    
More wear than it "should" for a 4-month-old bike (which in "normal" circumstances would have about 500 miles on it), or more wear than the "allowable" amount (which is about 0.75% to 1%)? (You can buy an inexpensive "go/no-go" chain stretch gauge to measure this yourself.) –  Daniel R Hicks May 21 '12 at 2:07
    
200km a week is a lot. In 4 x 4 = 16 weeks you get 3200km. Even if you clean and lube, this is enough to make most chains go beyond end of life. Since most people usually don't ride this much, I think that's what the shop guy meant with "more than it should for a bike with 4 months". Twice that it should probably mean 2% "elongation", considering 1% as the reference that prompts for a new chain. –  heltonbiker May 21 '12 at 3:01
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