Do I need to remove the headset cups to convert threaded to threadless fork?

Question

I want to change a 1 inch threaded fork to 1 inch threadless fork, but according to some tutorials on the internet, the process of removing and installing new cups looks a bit difficult and requires special tools.
Do I also really need to completely change the headset parts to make this conversion?
It appears that both have some parts in common, such as the cups and bearings. So I would keep those and only change the threaded race and nut.
My bicycle current headset looks something like this

But I also have an old headset that looks something like this

Context: I started assembling a new bicycle myself from used parts just as a hobby, so I am not worried about performance / cost. Also, the bike shops are all a couple hours drive from my house, so I want to avoid asking someone to do it for me
If you can share a few more considerations about changing forks, it would be appreciated.

Nathan Knutson · Accepted Answer

In most cases, you'll need a complete new headset. The pressed cups can look similar but that doesn't make them interchangeable. Usually some aspect of the sealing or fit wouldn't work right if you did that, even if both headsets used the same bearing retainer.
A few companies use literally the same pressed parts for both threaded and threadless models, and if you have that sort of headset and know it, then yes you could replace everything above the upper cup and it would work. That is an outlier situation and is the only time it's reasonable to do this.

Armand · Answer

Sadly, it looks like you probably DO have to change the pressed cups.
As mentioned by @NathanKnutson, the details are what matter. You indicate that your headsets are "something like" those shown in the diagrams. In those diagrams, the upper (threaded) headset uses loose/caged ball bearings, while the lower (threadless) headset uses 36 degree angle "angular contact bearings". The ACBs won't fit the ball bearing races, nor will the ball bearings fit the ACB seats.
If your existing and unused headsets use different bearing types (as with the ones in the diagrams), you'll have to change the pressed cups to match.
You hint at the headset change being in service of a greater goal -- what is that goal? Why not post another question regarding what you're trying to achieve overall?

JoeK · Answer

The answer Nathan gave is fine, but you say you want function right away.
The bottom cup and bearing is fine. You will need to move the crown race from the old fork to the new fork but the bottom bearing has no relationship with the top bearing other than they turn at the same time (and share "preload").
For your top cup, the majority of loose/caged balls use the same diameter balls and the same diameter cages so you can test (without the fork) that your new threadless top race fits together and runs without fouling. If you can achieve smooth roataion then the headset is safe to use. You may not achieve perfect sealing (though you can use pleanty of grease to help with this and the top bearing doesn't need to endure road spray like the bottom cup.
If you can't achieve a free/smooth turning bearing, without play, with your mix of parts you will need to replace the headset (at the very least the top cup, races, etc).  The steering is very important to your safety!
I hope that helps you to make the bike functional to try out.

Do I need to remove the headset cups to convert threaded to threadless fork?

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