how do i determine headtube size
looking at getting a new fork and i need the headtube size on my bike. is it the measured from the inside of the tube or the outside of the tube?
The outside diameter of the fork steerer tube is the basis for head tube sizing.
This means that (for example) a 1 1/8" fork will require a larger internal diameter headtube than 1 1/8" to accommodate a headset as well. Exact sizes can vary, so try looking up the model year of your frame to find out what it needs. If nothing else, see if you can have an LBS figure it out for you, if nothing else they probably have headsets for each size that they can try.
Originally Posted by Svr
*Assuming* you have a threadless headset:
1. Remove the top cap from your stem;
2. Measure the diameter of the inside of the stem;
(You should see the steerer tube of the fork ending a few millimeters below the top of the stem.)
3. Replace the top cap.
There is no need to make it very tight, since it's job was finished when the stem bolts were tightened.