It is a Trek 1500 (road).

It has a 105 triple Octolink but never use it (third or small chainring) and since I have a couple other bikes I thought I would use this to learn a bit more about bike repair/replacement of parts.

I would like to put a double on it, but I do know that I have to replace some of the other components as well.

Have tried the shimano and trek websites but cant seem to find it. Should I just remove them and measure?