Hi, Is there a 21mm stem adapter on the market? I can only find 22.2mm sizes and I have a 1972 Nishiki with a 21 mm stem which I want to add more modern handlebars to. Any help you can provide would be appreciated.

If what you have is an old 21.1 mm stem, you have only one option. This bike shop has the only known supply of the adapters, and as the proud owner of two, I have tried my best to find them elsewhere.

there are .883" adapters, a common size for older BMX bikes ,
the metric equivalent may be what you seek, converted to MM.

Thanks for the tip. I ordered an adapter yesterday.