Hello, first question here..
I have an older mountain bike and was thinking of cutting the head tube off it, and welding on one sized for the newer 1 1/8" steerer tubes and related hardware as well. Actually I would set everything up, then have it professionally welded.
My question is, one would this be worth doing?
And two, what are the ID and OD dimesions of the tubing I should use, or can I simply buy something that is sold specifically for this purpose then just cut it to my desired lengths?
Unless it's a VERY high-end bike (and made of steel, NOT aluminum), it'd cost more than the frame is worth. And if it IS a high-end frame, you'd destroy any value it has. Not to mention the nuts-and-bolts issues (alignment, geometry, etc).
I was halfway thinking about the fun of doing it more than anything. One of those learn as you go things. My uncle runs the steel shop of a commercial construction company that works throughout the US, so the welding would be next to nothing if I had the setup ready to go.