Hey guys,
I've got a Trek 1400 that I came upon for a good price. I'm enjoying it so far, but I want to swap out my handlebars to a more comfortable shape. Ran into a little problem after I ordered the bars, as can be seen in the pictures. The circumference of the new bar is half an inch bigger than the existing bar and thus won't fit into the existing tube. Basically, my question is this: what other parts do I need to make this work? New fork and steering tube and related parts? Or could a threaded fork compatible piece be found? As you more experienced techs can tell by now, I'm totally new to this. I could wuss out and take it to the local bike shop, but I won't learn much by paying someone else to do the job for me, nor is that much fun. Let me know which direction I should go on this, please.

Looks like you would need a quill stem that has the same clamp size as your new bars. Or you could get different bars to fit the clamp you currently have.

You bought handlebars which are the now-common "oversize" 31.8mm diameter at the clamp. What's in your bike now is the old-standard 26.0mm diameter.

You would need a different handlebar stem. I don't think there are many quill stems that fit the oversize handlebar. I would recommend returning the oversize bars and buying the correct ones for your bike.

My 1992 Trek 1420 (triple crank version of the 1400) came with 25.4 mm SR bars and stem so it's quite possible the OP's bars/stem are that size too. In any event, as you said, finding a quill stem for a 31.8 mm bar is not going to be easy.

One possibility is to get a threaded to threadless adapter ( like this: Nashbar Stem Adaptor - Road Bike Stems ) and buy a threadless stem to match the bars.

I would do the threaded to threadless adapter and a new modern stem and call it good.

In my opinion, why add new parts to try to work around it? You're gonna have to wait on a parts order anyway to get a threaded-to-threadless adapter. May as well just spend that waiting time on getting the right-sized handlebar, and meanwhile you can return the wrongly-sized one and save money.

Since the 26.0mm and 25.4mm sizes are so close, I think you should confirm with a caliper. If you're very careful measuring, you can wrap a tape measure around and divide the reading by pi to find the diameter.

circumference of a circle = diameter x pi
...and therefore...
diameter = circumference / pi

That will tell you the right diameter you'll need, in theory... but with only .6mm margin of error, that'd be enough to scare me away from wanting to try to measure it by hand in that way.

One of the photos shows a "Coda" (Cannondale) brand handlebar. I'm not certain, but I'll bet these never came in 25.4mm diameter.