this is what comes up for a 400f on hondarestoration.com. Looks like it's 2.5 mm id. unavailable, from this site. That doesn't mean you can't find it somewhere. So, i'd google the part number and start " digging" from there. If you're new at this , you learn how to scrounge around for supposedly unavailable parts. Try davidsilver spares, 4 into 1, etc.