Thought I'd put this here too as it really applies to a lot of potential builds and this type of bearing is simple and cheap. They're flanged split PTFE bearings and are just a quid or two each from ebay. They are 1mm thick so an 18mm o/d x 2mm wall tube will take a 14x12x7mm bearing and use a 12mm bolt. They make kingpins very easy and quick to make and are very smooth in operation. They press in with a vice or moderate use of a hammer and I can't see me using steering tube and headset bearings again.