Head tube-How to DIY your bike with good carbon frame

Head tube/ Steerer tube diameter

This is how your fork fits into the frame’s head tube. Your stem and handlebars clamp to the end of the steerer tube.

Older road and mountain bikes: Straight 1 1/8” steerer tubes, or 1" for vintage models. 
Newer road and mountain bikes: Tapered steerer tubes — 1 1/2” at the bottom tapering to 1 1/8” at the top of the head tube is common for most mountain bike forks. Road, gravel, and, cross forks are usually 1 1/4" at the base and taper to 1 1/8". 
Odd sizes: Straight 1 1/4” or 1 1/2” steerer tubes seen on Giant overdrive and older Cannondale Lefty.