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My crankset is worn down and needs to be replaced. Can I take the intact crankset from an older, broken bike and mount it on my newer bike? If it matters, both cranksets are Shimano (different models), have 3 chainrings and 48 teeth on the largest one.

I'm mainly worried that the horizontal distance between the chainrings might be different, which would make accurate shifting impossible. As far as I can tell they seem to be equal, but it's pretty hard to measure them accurately. Also I imagine there might be other things to take into account which I'd never think of.

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  • Crankset shouldn't really wear out. The chainrings are consumable part and can be replaced. – el_oso Jun 15 '20 at 19:30
  • @el_oso Unfortunately my chainrings are clinched together and can't be replaced. – MaxD Jun 15 '20 at 19:31
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    Lower end mountain cranksets often don't have removable chainrings – Armand Jun 15 '20 at 19:50
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No, cranks are not generally always interchangeable, but you probably can transplant the crank from the broken bike.

First, do both bikes have 6, 7 or 8 sprockets in the rear cluster? If so, they use the same chain width and the spacing between the chainrings will be the same. Same deal if they both have 9 sprockets. (As you mention the chainrings are riveted to the crank I think you probably have a lower number of sprockets).

The cranks need to have the same chainline, but MTB triple cranks almost always used 47.5-50mm so they should be compatible in that regard.

Older or lower end 'three-piece' cranks use a bottom bracket that includes the axle and are threaded into the frame, the crank arms fit into a square or splined taper on the end of the axle. Newer 'two-piece' cranks incorporate the axle into the drive side crank and fit into bearing cups either threaded or pressed into the frame.

As you mention your crank has fixed chainrings it's certainly of the three-piece type. I assume the donor bike is the same. If you tried swapping the cranks only you may run into trouble with the axle interface being different. Also different model cranks generally require a different length axle to get the correct chainline, so the bottom bracket axles may be different lengths. You can get around this by swapping the crank and the bottom bracket together

The only other stumbling block is if the bottom bracket shells are different widths, On older MTBs 68 and 73mm wide shells were both used.

Check that the donor bottom bracket bearings are not worn out of course. Note that special tools are needed to pull the cranks off the axle tapers and to unthread the bottom bracket cartridge.

Here are a couple of Park Tool videos that include removing and installing 3 piece cranks and bottom brackets.

https://www.parktool.com/blog/repair-help/how-to-remove-and-install-a-crank

https://www.parktool.com/blog/repair-help/bottom-bracket-removal-installation-threaded

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Besides the horizontal spacing between rings (determined by the number of gears the crank is designed to work with on the rear wheel), you also need to compare the bottom bracket compatibility. First, the type (square taper, octalink, hollowtech, etc), then the dimensions (especially for square taper, different cranks are designed for different length BB spindles).

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As long as they both use the same interface (likely square taper in your case) and are from the same bike type (road vs MTB; MTB cranks are usually spaced wider), it should work. It will not be unsafe to use. Worst comes to worst, you try it and see if the shifting functions properly or not.

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  • BB shell width and BB spindle length need to be checked as well. – Armand Jun 16 '20 at 15:20
  • @Armand Shell width is part of the “road vs MTB” step, as long as OP transplants the cranks from the same style bike it should be fine. With the bike description they’ve given us (riveted crankset), I’d imagine they don’t have too many rear gears, so the spindle length probably isn’t extremely critical. Hence my recommendation to just mount them and see what happens. – MaplePanda Jun 17 '20 at 17:47
  • I know MTB commonly use at least 68mm and 73mm BB shell widths, which is why I was suggesting checking shell width even if the donor and recipient are both MTB. Riveted cranksets are common on the low end even for 7/8 speed rear gear drivetrains. In any case, it's easy to measure the shell width and spindle length on donor and recipient and get an idea if there might be an issue. I've upgraded a number of cartridge BBs + cranksets and been surprised at how much the correct spindle length can vary with different cranks (e.g. 113mm->121mm). – Armand Jun 18 '20 at 19:09
  • @Armand I didn't know there can be that big of a difference. MTB's all use 73mm BB shells (other than DH bikes using 83mm, which OP doesn't have), so I wagered there can't be too big of a difference. As you said though, it only takes a second to measure. – MaplePanda Jun 19 '20 at 3:21
  • I think I'm not as modern as you :) The 2 bikes I have with me here are a 96 Trek 950 with 73mm BB shell and early 2000s Marin Eldridge Grade with 68mm BB shell. Elsewhere I've got a 2008 Giant Yukon with 68mm BB shell. Perhaps in the next decade I'll move up to a mid-2010s bike! – Armand Jun 19 '20 at 3:45
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Likely yes, but it depends on the details. Since your chainrings are not removable, you likely have a lower-end mountain triple crankset; having 48 teeth on the largest ring is consistent with this.

1) The main consideration is what type of crankset you have. You most likely have a "3-piece". That has two removable crankarms (drive (chainring) side and non-drive side) along with a bottom bracket that contains the bearings and spindle. The BB is attached to a hollow shell in the frame, and the crankarms are attached to the BB spindle.
You almost certainly have a threaded shell with "English" threading, and the spindle most likely attaches to the crankarms using a "square taper" interface (although it might use a splined interface instead). The shell is probably either 68mm or 73mm wide. 2) The key consideration in replacing the crankset is that the chainrings on the new one are the same distance from the centerline of the bike as the old ones, so the "chainline" remains the same. This depends on both the dimensions of the chainrings/crankarm and on the dimensions of the BB.
In your case, you want to change both crankarms/chainrings and the BB, so you will have to look up the Shimano documentation for both the old and new crankarms/chainrings and BB. You want to make the final BB+crankarm/chainring "chainline" the same as your old one (probably 50mm). 3) More specifically: a) assuming you have an English threaded shell in your frame, you need to make sure the new BB fits the shell (i.e. English threaded, and 68mm or 73mm according to your frame shell width). b) then, you need to make sure the BB spindle length is the correct size so that when the crankset arms are mounted, the chainring chainline will be the same as for your old set. Shimano docs will indicate what spindle length is needed for your specific "new" crankarms/chainrings. c) you also need to double-check that the crank arm length is the same between old and new. 175mm is pretty standard, but 170mm also shows up for people with shorter leg length. d) If the chainrings are worn out you almost certainly also need a new chain and a new rear gear cassette. Your new chain length will have to be adjusted to take account of the difference between the number of teeth in the biggest and smallest front chainring, and the difference between the number of teeth in the biggest and smallest rear gear.

Obviously, this is just a rough overview without the specific details of your situation. However, you should also read/watch about "BB removal and installation", "crankarm removal and installation" "chain replacement and sizing" "rear cassette replacement" "pedal removal and installation" "front derailleur adjustment" "rear derailleur adjustment". It's not a trivial operation you're doing, so I would strongly recommend having an experienced person to help or be on call, or have a professional mechanic at your local bike shop do it. There are some specialized tools involved, and improper installation can damage your bike or lead to injuries while trying to ride it.

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