Like this: https://www.sjscycles.co.uk/stems/genetic-quill-stem-adaptor-1-inch-quill-to-1-18-inch-threadless-adaptor/
Exception would be if you wanted to use some rare, esoteric mtb stem from the 90s, then you would use something like this (very unlikely) https://www.condorcycles.com/products/deda-quill-to-a-head-stem-adaptor?variant=12521459679315
Thanks. I thinknits the first one I'll be after. I started to question which one would be the correct size when they mention internal diameters of 22.2, but I think it's the first one that will be correct.
@Brum started
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Like this:
https://www.sjscycles.co.uk/stems/genetic-quill-stem-adaptor-1-inch-quill-to-1-18-inch-threadless-adaptor/
Exception would be if you wanted to use some rare, esoteric mtb stem from the 90s, then you would use something like this (very unlikely)
https://www.condorcycles.com/products/deda-quill-to-a-head-stem-adaptor?variant=12521459679315