Are all threaded headsets the same size
Threaded headset sizes are designated by the outer diameter of the steering column. This can seem confusing, because the head cups do not measure the named standard. The threaded standards are 1 inch, 1-1/8 inch, and 1-1/4 inch headsets. The various standards are generally not interchangeable.
How do I know what size my headset is
You will need to measure (in millimetres) the inside diameter (ID) of the top of the head tube (above pic) and the bottom of the head tube (pic below). Measurements are rounded to the nearest millimetre. Example: a tapered frame with a 44mm upper ID and a 56mm lower ID.
Can you put a threadless headset on threaded forks
CAN I USE A THREADED FORK WITH A THREADLESS HEADSET? Possibly. … The steerer protrudes enough above the leading edge of the head tube for the top race, washer, and locknut to comfortably fill the space between the top of the steerer and the headset cup in the head tube.
Can you replace a threaded fork with Threadless
It is also possible to take your threadless fork into a bike shop and cut threads into it, this will likely cost you more in labour than replacing the headset. You also haven't mentioned the material of the work, and this process will depend on that too.
How do I know what headset bearings I need
If you can see a headset cup outside of your frame, even just a flange or lip (as in the photo above) then your headset is Press Fit. If you can't see a headset ie. it's entirely sitting inside the frame head tube, then your headset is Integrated.
Are all headset bearings the same
All bicycle headsets work in a similar way – there are two bearings, one for the top and one for the bottom of the headtube, and your fork's steerer tube passes through the headtube with lower and upper bearings.
How are threaded Forks measured
Begin by measuring head tube length. Next, determine stack height. The stem is not included in figuring stack height at all. If you are measuring stack height, assemble the lower part of the headset with bearings in place.
What is my headset size
The dimension of the headset is further defined by codes identifying the diameter of the fork steerer at the top and bottom of the head tube. The upper section of the steerer is identified by its diameter, and in the case of threaded steerers by the thread used for that steerer.
Do all forks fit all bikes
Will any fork fit my bike? No. When replacing forks it's key to know your head tube diameter as you will need to pick a fork with a matching steerer tube.
Can you change a threaded headset to a Threadless
No. Most threaded headsets are 1" and most threadless headsets are 1 1/8". So it physically won't fit. The best you could do is run a converter like this to run a threadless stem, but that's not going to do much for you.