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I have a 2015 Giant SCR2. That has Shimano Claris. Outdated by now 2x8. I’m thinking of an upgrade to Shimano 105 5800 or Tiagra.

Which one will fit and do I need to change the wheel?

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Regarding ‘which [groupset] will fit?’ Bicycles are generally so standardized that you can swap out groupsets easily and only have to deal with one or two areas of compatibility.

In your case you need to consider the rear wheel. The freehub body for 11 speed compatible hubs is 1.8mm longer than those for 8-10 speed. You’ll have to examine your hub too see if yours is 11 speed compatible. If there is a 1.8mm spacer behind the 8 speed cassette the hub will be 11 speed compatible.

Generally you also need to look at the bottom bracket for crank compatibility. As you have a ‘3-piece’ crank with cartridge bottom bracket you can easily replace that with a Shimano HollowTech external bearing bottom bracket, which all Shimano road cranks are compatible with.

Rim brake groupsets are relatively inexpensive, but most of the time it’s not worth upgrading the groupset on a bike. Often it’s better to sell the current bike and purchase one that is a complete upgrade, especially when you factor in the cost of special tools you need, or paying for a bike repair shop to swap out the components.

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