The obvious first thing to check is that you have a 26.4 handlebar. If so you can probably get away with a shim. Or possibly using a center punch to "peen" some spots inside the stem clamp area to give it some tooth. But with that said, these pieces may just be worn out and ready for the trash bin. A bar and stem is no place to put economy in front of safety. NOS 1A's are still available and the black ano ones go pretty cheap. They are easily stripped and polish up beautifully. Something to consider.