Hello,

I have medial pain in the left knee, and a bike fitter recommends to change crank arms from 172.5mm to 165mm. The bike I have is a 2016 Giant Fastroad SLR 1. The specs sheet simply says crankset is an "FSA Omega, 34/50" and bottom bracket is "FSA, Press Fit." I googled for more info and found that the bottom bracket may possibly be BB86/PF86.

Since the bike is new and for cost, I want to change only the crank arms. I also want to search Craigslist used parts first. So, I would like to know if I need to stick with FSA Omega to avoid incompatibility, or any FSA crank arms would work? Also, if you know of any cheap online place, I would love to know, too.

BTW, I'm very new to bicycling (last June/July). So far, I have lubed bike chains, but nothing else. I have watched Youtube videos on removing and installing FSA cranckset, and the process simple enough that I'm sure I can do it. However, if you have any tips or suggestions, please, let me know.

Thanks in advance for your help

Hoping I understand what you are asking here:-

True fully if you are new to biking & do not have any tools & very limited knowledge, no choice take it to your local bike shop explain the situation, basically you cannot change crank arm length without changing the whole crank out i.e. left & right side, to your new desired length, & make sure you end up the same crank teeth 34/50.

I cannot see any reason why you need to change out the BB ? others on the forum may know more detail.

If the crankset is BB30, then you may be able to replace just the crank arms if the new ones are compatible with current spindle.

FSA Omega Compact - FSA
Bottom Bracket Standards | Park Tool

Nashbar has that crank on sale today in 165 for $60.
FSA Omega Compact 50/34T Crankset

Looks as if Nashbar is only offering the MegaExo version, which won't work on a frame designed for BB30 bottom brackets, as I understand it. Unless there is an adapter for that.

OP says his is PF86. Internet says Bottom Bracket FSA Mega Exo.