The bars in what photo?

Older Cinelli stems are 26.4mm inside diameter as were Cinelli bars outside diameter from the same era.

Most older road bars from the last 20 years have been 26mm diameter but many have also been 1" or 25.4mm diameter, many Nitto bars for instance. You need to accurately measure your bar diameter in the stem clamping area to know what you have and then get a matching stem.

.4mm is .016" which is a lot more thickness than a beer can. You need to get a replacement stem which matches your current bar diameter or get some steel shim material which is .008" thickness and wraps completely around the bar but does not overlap. Probably safer to get a new correct diameter stem presuming the bars are 26mm diameter.