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).