Stems - All our quill stems have a 22.2mm diameter quill and our threadless stems are 28.6mm (1-1/8"). Most are either a 25.4mm or 26.0mm handlebar clamp. It is worth checking which you need before purchasing. Quill stems have height adjustment and threadless are fixed height. If you want your stem to be horizontal, you need to purchase a 17° or 73° stem.
Headsets - Either 1" threaded or 1-1/8" threadless. If your bike is low trail, you need a needle bearing headset to reduce shimmy.