I'm sure someone will be along soon to tell me I'm an idiot for saying so but if it's an old stem that's most likely been tightened/loosened many times, you may be okay. My beater has 26.0 bars in a 25.4 quill stem and they installed without any trouble at all. If it's a tight fit, you could always file down the inner edges a hair just to smooth them out and keep them from biting into the bars.