Originally Posted by
halfspeed
Be careful. It could have either a 25.4mm, 26mm, or even 26.4 clamp diameter. You might want to measure it with a caliper.
If the stem (aka gooseneck) is as old as the bike and it is a Cinelli, it's really likely to be a 26.4. But do measure the ID of the handlebar hole, you do need to get the correct size.
Road Fan