25.4 bars on a 26.0 stem...
 

and it fits perfectly fine. Should I be worried? I've had it a for awhile now and never it has become lose or anything.

it might creak like crazy. use a shim.

I've used a 25.4 bar with a 26.0 stem for two years without one creak or issue. No shim needed sometimes.

try this... grab your bars from the front of the bike and lift yourself off the ground with your arms.

if your bars move you should shim it.
if your bars don't move, you don't have to do anything to it.

be sure you grab it at the points furthest from the clamp at the stem while trying the above proceedure.

I'm dealing with this problem right now. I just bought Soma bullhorns and they're 25.4mm and my stem is 26mm. I couldn't get my stem tight enough to hold my body weight. I went to my LBS and they recommended buying a shim or cutting up a beer can to use as a shim. I went the beer can route and it's working pretty well. Make sure to place the aluminum pieces on the top and bottom of where the stem connects to the bars(where the screws are).

Just shim the dang thing.

Over-stressing an aluminum stem and/or bar isn't a good idea. If it fails, it will be bad an you will get hurt.

Stress risers.

I have new 25.4 North Road bars and an old 26.0 Nitto stem. So when I bought the bars, I bought a Nitto shim, too. But the other day when I went to put on the shim, it wouldn't slide on. There isn't enough extra space to slide the shim into place! So I installed the bars without the shim and tightened the stem nut. The new bars seem secure and, at a quick glance, there's still a few threads of travel left on the stem-tightening bolt. Just wanted to check with you-all. Am I missing something? Is it odd or dangerous to have a 26.0 stem accept and hold a 25.4 bars without a shim required? Many thanks.

I think Asahi cans are NJS, try one of those.

Yes, it is potentially dangerous to do this with a quill stem, because you are deforming the stem and aluminum is not very ductile, so it could develop cracks that could expand suddenly. This could result in a sudden failure that would cause the bar to separate from the stem and your teeth to separate from your face. This might happen tomorrow, or ten years from now or never. How lucky do you feel?

+1

+2

Tejano Trackie: Thanks for your note. Last night, after I posted my message, I decided it was best to install the Nitto shim, so I did it. Had to gently pry open the stem clamp to get the shim to fit, but it wasn't too difficult. I feel better now. I value my teeth!

It should be fine.

I think Kirin lager is the oldest Japanese beer. Says something like that on the can. Keep it classy.

ElliotSF: If you want to be super extra sure, you should totally trade with me. I have two Nitto Technomic stems, 25.4 with moderate reach. In search of 26.0 Nitto stem of similar dimensions. For everyone else: Don't stick handlebars into non-matching stems if you value your face. Shims are acceptable, but you should always check them every few months, grease, etc. to prevent seizing. Over-clamping does not work.