Originally Posted by
Harcati
Forgot to ask tsl... will I need to change the chain if I go Tiagra? Currently running a Shimano 105 chain.
No.
The chainrings on both cranksets have the same number of teeth, so you won't need a longer or shorter chain.
As far as "compatibility" goes, all Shimano 10-speed stuff is freely interchangeable with other Shimano 10-speed stuff.
Some parts are also freely interchangable between different speeds. For example, I recently upgraded my 8-speed Sora bike to 10-speed. I replaced the levers (105 5603-series), the cassette (Ultegra 6700-series), and the chain (KMC DX-10 not even Shimano). Still running the "8-speed" Sora crankset, FD, and RD.
They spread a lot of FUD around hoping you'll fall for it and buy extra parts.