Hi Vincent

None of our esteemed mentors have replied for four days; so here are a few thoughts from me.

As a first step into the world of framebuilding I would like to start by welding chromoly threadless stems, in different clamp diameters. I made some research and think I have kinda figured out the tubing selection, but I would need some reassurance.

1. What is generally the O.D of the extension tube? From what I have gathered it seems 1-1/8" is the way to go, independently from handlebar clamp diameter.

yes, this is a good place to start.

2. The most suitable tube I found for a 25.4 handlebar clamp tube was 1-1/8" (28.6mm) O.D, 1.009" (2.62mm) I.D and 0.58" (1.47mm) thickness. My question is, what dimensional tolerance do I have for the handlebar clamp diameter? Could that extra .22mm cause problem? Other option I found was 0.995" (25.27mm) I.D with .065" (1.65mm) thickness.

that .22mm will cause problems. the 1.65mm wall, and a reamer, is a better solution.

3. Is .058"-.065" (1.5mm-1.6mm) thick enough for stems? Would .049" (1.25mm) be suitable for the handlebar clamp tube only? I would like to make 26.0 clamp stems too, but the only option I found is 1-1/8" (28.6mm) O.D, 1.0270" (26.08mm) I.D and 0.49" (1.25mm) thickness.

i personally would start and stay with thick stuff. if once you're super confident and competent there, you want to begin exploring, thats up to you. Again, a reamer and/or small lathe is your friend here

4. Where would one order tubes and binders? I did manage to find tubes on Aircraft Spruce and other websites, and M6x1.0 binders on Nova Cycles Bike Fab Supply (althought I'm not sure they would work for every handlebar clamp diameter?), but I'm still curious if you have other resources.

google is your friend here, paragon machine works, bikefabsupply , framebuildersupply.com

Thank you for the help

also, we use first and last names attached to each post here, its a rule.

post up some photos of how you get on!