I dont know.

I've seen a stem failure in person. The person ended up with broken collar bone and a few knocked out teeth. Whether or not this was caused by putting a 26.0 bar into a stem 25.4 or putting a 26.4 bar into a 26.0 stem or some other cause, I dont know -- the bolts ripped out. But why risk it when one could easily flip either component and avoid any future trouble? It seems most injuries I've encountered over the years that were caused by mechanical trouble were rooted in hacks that were justified the same -- "oh I'm sure it'll be fine." A friend of mine broke the bracket on his road hoods and fashioned a new one out of scrap metal -- it was fine for awhile until it suddenly snapped while riding. The brake lever fell off and was dangling by the cable and got caught in his front spokes. He endo'ed over and landed on his shoulder -- dislocated it and destroyed his front wheel and fork. Of course, he didnt have insurance and ending up paying about $5k in hospital bills. He would have better off just buying a new set of Tektros for $25 instead.

I am sure that people will be lucky and get away with 26.0 into 25.4. I am also sure that some will not be lucky and end up wrecking. I think when it comes to bicycles, it's always better to reduce risk as much as practically possible and simply using the correct fiitting components together is an easy way to reduce risk without sacrifising much.

I reamed out a quill stem once in a machine shop for just this situation. Rode it for 5 months without any problems. The <1mm amount of steel that I lost is not going to be effected at all by the small amount of pressure I put on the bars, even when out of the seat.

EDIT: Didn't realize it was an open faced stem. Bad Idea, enjoy your new stem.

...I'd recommend doing that over bolting two pieces together that do not quite fit.

I've built a lot of chopper/tall/freak bikes that had some pretty shoddy construction, but this is one thing that I would not do.

You can't just get away with a fit like that. .6mm isn't a small difference when it comes to parts like stems which are designed and machined to very specific tolerances.

Plus, it isn't going to fail on a leisurely ride down to the beach, it will be when you are cranking on it and will probably end up face down on the tarmac with your bars in your hands.... wishing you would have listened to the advise given here.

well said.