Almost certainly your threaded fork is too short to take a threadless stem and many would say don't clamp a threaded section of fork anyways. You can get quill adapters to take a threadless stem. https://www.bing.com/images/search?q...cles&FORM=IGRE If you're determined to totally convert you'll need a new fork, stem, spacers and possibly some new handlebars as most threadless stems are made for larger diameter bars in the clamp area but there are still several stems still made for smaller bars usually found with quill stems.