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I came into possession of an old Mongoose 3.5 that I need to buy a chain for. Unfortunately, I know very little about how to properly purchase the correct chain for a bike I can find very little information for. No bike shops around here are available to service it.

The bike has a rear cartridge with 7 speeds and a pedal cartridge with 3 speeds.

I am also not sure if I need to replace anything else. The rust on the derailleurs looks like it can be scraped off and I can clean the rest of the bike.

bike side view

mid view

rear view

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    A possible hack here is to obtain a bottle of oxalic acid from a paint shop (often labeled "wood brightener" or some such). Use a toothbrush to apply the stuff to the chain, soaking it as best you can. This will dissolve an astonishing amount of the rust, and may make the existing chain serviceable again. (Note: Be sure to rinse the chain with water, allow to dry, and then oil it well, if you use this treatment.)
    – Daniel R Hicks
    Jun 23, 2020 at 22:48
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    Cables will probably need a lube, maybe need replacing. If brakes are poor and cables have been replaced, new brake pads will probably help. If the chain moves, lube it and go for a ride and find out what else needs doing before spending money.
    – mattnz
    Jun 23, 2020 at 23:43
  • A minor correction for clarity: under "pedal cartridge", you actually have 3 chainrings, aka triple crank. For "rear cartridge", you have a 7-speed cassette or freewheel (those are actually two different things). On bikes, many ball bearings are encased in cartridges (i.e. sealed cartridge bearings, or a cartridge bottom bracket), or some of us use CO2 cartridges to reinflate tires. This is offered not out of pedantry, but to help communicate with shops or on other forums.
    – Weiwen Ng
    Jun 24, 2020 at 16:49
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    @DanielRHicks If you're taking the time to go to the hardware store, you'd be better off with a proper rust removing agent like Evapo-Rust.
    – MaplePanda
    Jun 24, 2020 at 23:19
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    @MaplePanda - They don't tell you what's in their "proprietary formula", but I'm pretty sure it's largely oxalic acid.
    – Daniel R Hicks
    Jun 24, 2020 at 23:27

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All you need is a 7-speed chain. You’ll have to shorten it by a few links after purchase (they’re sold excessively long for funkier gear arrangements), which should be done with a specific chain tool. This tool will also be needed in order to remove the current chain.

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    Yep, should be a plain vanilla 7-speed chain. Chain tool needed, and you may want to obtain some sort of "quick link" to do the joining, as using the chain tool to join is kind of futzy.
    – Daniel R Hicks
    Jun 23, 2020 at 22:44
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    I'd definitely try to get a new chain with a "quick link" - a removable link that designed not to need a chain tool to remove. You're going to have to remove the chain eventually. You'll still need a chain tool to cut the new chain to the proper length, though. (Note that this doesn't mean you can remove a quick link with your hands - you probably need a pair of pliers or similar to remove one - don't forget that in the tool pack you have for looong rides. Ask me how I know...)
    – Andrew Henle
    Jun 25, 2020 at 1:02
  • If going for the "quick link" approach, one should add few quick links to the emergency tool kit one carries around. Actually, even 7-speed chains typically require special pins, so even for non-quick-link chains, one should purchase few of those special pins. The special pins are lighterweight addition to the emergency tool kit and take less space. Of course all this assumes the emergency tool kit has a lightweight mini chain tool.
    – juhist
    Jun 25, 2020 at 15:36
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You need a chain suitable for a 7 speed drive train. The manufacturer may specify it fits 6,7,and 8 speed or any combination of that. As long as it says 7 speed it will work. You will also need a bicycle chain tool. This tool separates the chain so it can be removed. It will also be needed to make your new chain the correct length. The easiest way is to make the new chain have the same number of links as the old. This is critical, the same number of links is not necessarily the same length. As the chain wears it gets longer but the number of links will remain unchanged. Reattaching the ends is easiest using a quick link which may come with the the new chain or be purchased separately. So at this point you have to decide to invest dollars in the tool or just have your local bike shop do the job for you.

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  • or even if it just says 8-speed.
    – Noah Sutherland
    Jun 24, 2020 at 23:20
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    Note that buying the tool is probably cheaper than paying a bike shop to do the work - even just once. And you'll wind up using a chain tool often if you ride a lot.
    – Andrew Henle
    Jun 25, 2020 at 1:03
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It is quite possible that the rear gear block (cassette/freewheel) will need to be replaced along with the chain. The reason for this is that the chain stretches over time as it wears. When this happens the teeth of the gear block wear down to match the new distances between the links in the chain. Then they don't match the new chain when it's replaced.

Whether or not this is necessary depends on how much wear the drive train has had. After you change the chain, you can recognise this problem if the new chain slips over the gear block when you apply a lot of force (e.g. stand on the pedals). In extreme cases the shock of this can cause a link in the chain to break (this was my experience).

Also looking at your photos, there's also rust on the block itself.

You might be OK, but be prepared for this extra work - you'll need a tool to get the back gear block off the wheel, and you may need a lot of force to get it free.

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    But in this case the chain is simply rusted because it was left out in the rain without proper lubrication. The cogs are probably fine.
    – Daniel R Hicks
    Jun 24, 2020 at 12:30
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Shift the chain to the big chainring and then pluck at it, pulling it forward from the middle of the chainwheel. If it pulls out enough so you can see light between the chainring and the links of the chain, the chain is probably stretched out enough to need replacement.

At any rate, you will need a chain tool to "break" the old chain and remove it from the bike. Then, if you decide to re-use it (if it isn't too stretched out) you can soak it for a few hours in a coffee-can full of orange soda. That works pretty well to dissolve rust; not as well as more expensive stuff, but tolerably. A wire brush may help.

(Oh, and if you have a bit of extra bikechain lying around, practice with the chain tool. )

If you buy a new chain, go for an ordinary chain for a 5-8 speed cassette. You can get this kind of stuff online easily. If you get one with a so-called "powerlink" you'll have an easier time putting it on the bike. But you'll still need your chain tool because they always ship new chains with too many links and you'll have to remove a few.

Others have said you may need to replace the cassette (rear gears) and chainrings (front gears) if the bike has been ridden hard for a long time with a stretched chain. Happened to me once. Pain in the neck. May or may not cost more than the bike is worth.

If you get this chain situation fixed and you decide you like the bike, consider changing the brake pads, brake cables and housings, and shifter cables and housings. In the 21st century you can watch online videos showing how to do all this stuff.

I know all this because I do a bike clinic every year with the kiddos at our local public housing project. We pick through the bikes -- mostly cheap mart bikes -- that the cops picked up, and fix up the best ones.

Have fun! It's really cool to be able to care for bikes.

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I was going to suggest vinegar (acetic acid) but this video very elegantly addresses a bunch of rust removal options including vinegar: quite interesting

You'll need to let it soak, then brush it clean, rinse it, dry it, and lube it, all quickly enough that you don't let it start rusting again! You should probably buy a new chain anyway, but you might get that one going in the meantime. I've used vinegar before, it's slow but I've been impressed.

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