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Reference of the chain: KMC Z51
Reference back freewheel: Shimano TZ21 14-28 Teeth 7 Speed
Reference of the bike: BTWIN Rockrider 340

I recently acquired a second-hand bike and decided to repair it: The chain and the freewheel cogs are in very poor condition and rusty as you can see in the pictures below:

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The bike has not been used for a year by the former owner. As I don't have that much knowledge about it;

I was wondering: Should I change the chain or repair it? as for the freewheel, can I even repair it?
If I have to repair them how can I achieve it?

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    The tyres are definitely past their prime. Good enough for testing but replace before serious use. Check also carefully if the brake cables are still good. Might be rusted inside the outers where you cannot see it.
    – gschenk
    Commented May 8, 2019 at 15:43
  • I have had success cleaning up this degree of rust by spraying with oxalic acid (sold as a "wood brightener" at paint stores). Do this before you oil it and then oil after the ox acid dries.
    – Daniel R Hicks
    Commented May 8, 2019 at 19:52
  • Clean the chain and cassette with WD-40 (best is to remove them), wash them and lube them (never to much lube).
    – Jean-Baptiste Yunès
    Commented May 9, 2019 at 5:03
  • If you're out of options - cheap artificial vinegar - often available for picking is a really good, safe (for humans) deruster. Soak the parts in it for a few days - replace if the vinegar loses its smell and the rust just drops off. Follow up with WD40 and a good protective oil if it works.
    – Journeyman Geek
    Commented May 9, 2019 at 6:00
  • @JourneymanGeek Thanks for your comment, also I hear that the most famous famous soda (whose name I wouldn't mention) could help for that, is it true?
    – Ced
    Commented May 9, 2019 at 6:52

4 Answers 4

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As long as the crank arms, chain and freewheel turn freely and all as one. Without clunk's, slipping or sticking at any point there should be no need to replace any of it. The surface rust will not prevent it being fit for purpose and should wear off with use and some decent lubricant. You could always try a wire brush to remove surface rust without having to disassemble any parts and also just for reference if you do decide to change your "cassette" be advised that is a freewheel and not a cassette they are not interchangeable so avoid disappointment ordering a part that will not fit. The part you would need to order if you decide to is a 7speed freewheel not a 7speed cassette.

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    Good catch on the freewheel vs casette terminology! I usually call any cluster of cogs at the back of the bike a casette. Further info is available here for anyone else unfamiliar with the distinction: sheldonbrown.com/free-k7.html
    – SSilk
    Commented May 8, 2019 at 17:52
  • Thanks, just trying to help others on here, hoping it brings me good karma. And one day a knowledgeable member of this community might be able to answer or at least partly answer my burning question.. what make and model is my frame!
    – David
    Commented May 8, 2019 at 18:41
  • @David how did you spot it is a freewheel? Did you check the code on the large cog or is there something else that gives it away. I thought a freewheel something from the really old days, maybe 70s. Shouldn't expect it on a fairly new, albeit cheap, bike.
    – gschenk
    Commented May 8, 2019 at 20:48
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    There a quite a few tells to a trained eye it's easy to spot. Freewheels are still commonly used today on cheaper generic bikes such as department store etc. From kids bikes to town bikes or cheap mountain bikes most still use freewheel as it's a cheaper more basic component to manufacture. The alternative to freewheel is freehub and it has its own bearing and rachet mechanism where a freewheel relies on a simpler rachet and paul design. You will generally only find freehub on mountain bikes starting 500 upwards from new. Although I am sure you could find exception that's a good general rule.
    – David
    Commented May 8, 2019 at 21:00
  • @gschenk 7-speeds are usually freewheels. And, in this case, the model number, Shimano MF TZ21 is clearly visible. Bizarrely, the product photo on Shimano's website looks rusty...
    – David Richerby
    Commented May 10, 2019 at 9:53
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Try it!

Does the chain move at all when you move the cranks? If some parts are rusted to a solid chunk it is done for.

You may apply some wet lube, see if it moves. If it is rideable it may rub off surface rust. If the rollers are rusted inside and out doesn't flex properly replace it.

Likewise for the cassette. The rust might be superficial and it might work still.

I shouldn't push too hard wherever a snapped chain might get you into danger (eg sprinting in traffic). But that's probably not what you bought a rusted decathlon bike for.

Of course you may always replace it. A seven speed cassette and chain often cost in the range of 10 €$£. It's perhaps a good idea though to ride the bungee and see if it's worth investing into repairs.

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    The bike works in its current state, but it makes a annoying noise when the chain rotates so that's why I was worried and I preferred to come here for advice rather than take unnecessary risks, knowing that I'm ready to repair and restore it.
    – Ced
    Commented May 8, 2019 at 15:53
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    @Ced that chain noise is probably a sticky link. Try manipulating each link with your hands and find any that don't move as freely as the others, then flex that link in all three directions (yes, sideways) and relube it.
    – Criggie ♦
    Commented May 11, 2019 at 0:32
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Surface rust on sprockets and chain is not a problem, rust between chain plates and rollers is, as it prevents the chain from articulating properly.

Apply liberal lube to chain, spin cranks, wait to let lube soak in, spin cranks. Manually go around the whole chain flexing the links. If they are all relatively free moving you are OK. You may need to repeat the process. If there are particularly sticky links you can't get rid of replace the chain.

The other concern is chain wear and elongation (wear increases the play at each roller allowing the chain to elongate). Chain wear gauges are cheap, or have your local bike repair shop check it. If it's substantially worn replace it.

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Was working on a bike not quite that bad yesterday. Starting off the chain was frozen solid. Sprayed the chain (BEFORE oiling!) with oxalic acid, then waited a few minutes. Most of the links freed up nicely, but there were about 8 stubborn ones. Since I didn't want to wait overnight for the ox acid to work further, or remove the chain so I could soak it, I used two pairs of pliers to flex the stubborn links, then a dash more ox. Freed up nicely and worked smoothly after a treatment with the chain washer. (Which leaves me wondering if I could have used ox in the chain washer.)

(Note that I also used ox on the rusted seat post and several other components. Melts the rust nicely.)

Ran out of time working on a bollixed brake lever, so I didn't get to test ride it. Next week...

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