I presently have a 3x7 speed crankset with a sealed cartridge bottom bracket on my touring bike and want to upgrade to a new Hollowtech II bottom bracket and crankset, specifically, the SHIMANO Claris 3x8-Speed Road Bicycle Crankset (FC-R2030). My shifters are 3x7 speed indexed, and I will stay with the 7 speed rear cassette. Is there any reason why the 3x8 front crankset would not work?
A 3x8 crank will work with a 7 speed system. 7 and 8 speed chains are the same, as is the spacing between the chainrings.
The Claris crank specification can be found on the Shimano Manuals and Technical Documents site here. The FC-R2030 seems to only come in 50-39-30. If that is larger than you current crank you'll need to check that you have sufficient clearance with the drive side chainstay.
The specification says the FC-R2030 works with either a 68 or 73mm bottom bracket shell, so that will work whatever your bike has. Note it also says what bottom bracket you need (BB-RS500) that will fit either shell width.
The FC-R2030 provides a chainline of 45mm - that's the standard for road triples. You'll definitely want to check that that is what you bike runs as there's a chance it runs the mountain bike triple standard 47.5mm. Measure from the center of the seat tube to the middle ring. More info here https://www.sheldonbrown.com/chainline.html.
If you have 47.5mm chainline you'll need to look at a mountain triple crank instead of a road triple crank.
Reasons a different crankset may not work:
- Different bottom bracket width means it doesn't fit on your frame at all.
- Different chainring sizes can't be handled by the front derailleur.
- Inner chainring on new crankset hits the chainstay (or some other interference).
- Chainline differs (too far in or too far out), and the front derailleur can't move the chain to the chainrings.
- Different speed means chain is too wide or too narrow to work.
This is not an exhaustive list - there may be other issues you can run into. But these should cover most potential issues.
Reason #1 can be avoided by checking the specs on your bike and/or actually measuring the bottom bracket - including the threading used (most bicycles today with threaded bottom brackets use English threading).
Reason #2 is pretty obvious and easy to address.
Reasons #3 and #4 are less likely, but can be hard to predict and can be a real problem.
Reason #5 isn't going to be a problem going from 7 to 8 speeds, as the chain width doesn't change at all between those two. But chain width issues can can be a problem with larger jumps - like trying to use a 6-speed chain on a 10-speed crankset. A too-wide chain might not be able to engage smaller, inner chain rings, and a too-narrow chain might be able to get into places it's not supposed to and seriously break things. A wrong-speed crankset may also have shifting problems because the front derailleur throw is too large or too small for the distance between chainrings. An 11-speed crankset may have its chainrings too closely spaced for a 7-speed front derailleur to reliably shift - shifting to the larger chainring might be prone to tossing the chain too far.
For your specific case, though? The difference between 7 and 8 speed is probably the smallest difference possible. If you get a crankset compatible with your bottom bracket width (no road crankset on a frame with a MTB bottom bracket...), and get the same size crankset, you'll almost certainly have no significant problems at all - you may have to adjust your front derailleur some, and might even have to tweak your rear derailleur a tiny, tiny bit.
That's by far the most likely scenario. In your situation, I'd buy that crankset from the cheapest source I could find and trust that it would work.
But it's impossible to rule out problems, especially remotely via text. Pictures would help, as would exact bicycle model.