My Nitto Olympiade are 25.4 and my Shimano Dura Ace stem is 26.0. I bought shims for it but the stem seems to tighten to the bars just fine without them. I tried getting the shims in but it was a really tight fit. Can I run it without the shims are should I install them?

You should shim it. I've used aluminum foil adhesive tape before, some people recommend beer cans. You really shouldn't clamp an undersize bar in an aluminum stem.

you might need to open up the clamp and loosen the clamp bolt as much as possible to slide in the shim and then clamp down properly. we're talking about .6mm

but yes shim it. it may seem tight enough without but experience shows your handlebars will slip oddly at a bad time with a 25.4 bar in a 26.0 stem.

Thanks for the help folks.

Nitto makes a Shim for the purpose.. I was only able to get half,
of the 2 part shim in, so I called it good.

26-25.4= 0.6 in diameter, but, the shim is a radius for thickness,
so it's half that.. 0.3mm

0.3mm is a lot.

Shim it. Aluminum is prone to crack/shatter if bent (and closing by force a 0.6mm gap is quite a stress)

Shim it. Not only will you stress the stem clamp area but you will only contact the bar in two relatively small patches, making it likely that it will slip and be damaged. Handlebars which come loose are only funny in the cartoons.

I tried this, didn't realize it at the time and I don't remember the stem/bar combo but it was a 0.4 or 0.6mm diameter difference. I kept torquing down on the clamp because the bars were rotating and finally broke the Allen screw in the Cinelli threaded stem.

Now I have 25.8 bars in a 26.0 stem and that works fine though, with no shim.