I'm getting a new fork for a threaded headset. I am ordering the fork with the steerer tube precut. How do I know what length steerer I need. And what about the threaded length? Why don't they just thread the entire length of the steerer?

Who sells forks pre-cut? Is it used?

Anyhoo, steerer length=headtube length+ headset upper stack height+ lower stack height-2 mm

You'll need an inch or so of threads on it.

If you want to play it safe and get a fork that is a little longer than I brad's formula, you can always add a spacer. They are available in 1" or 1 1/8" diameters.
Been there, done that.

The reason threaded forks aren't threaded all the way down is that a quill stem's expander puts a lot of pressure on the interior of the steerer tube. Threads would weaken the steerer wall and it could fail at the point the expander pushes against.

The rule is that the quill should extend down far enough so the expander is below the threaded section.

Any threaded fork would come precut, because only the top inch is threaded so if you have to cut 1" off, it's not threaded anymore. You're correct on the formula, though.

Have you done that on a threaded steerer? Considering the headset races go into the head tube and the to nut screws onto that but can't bottom out on the steerer, I don't see how that would work.

Not true though. I got one just the other day- 10" steerer, 7" of threads. Cut to fit. It's a low end fork, but just an example.

No, they don't always come precut. They come with an extra long threaded portion so the fork will fit a range of sizes. You have to cut the steerer to length, just like a threadless setup, but you have to be careful not to screw up the threads.

The spacer goes under the top nut. I've seen 20mm of spacers used. It's a way to get the bars higher without using an extra tall quill stem. You could probably find an example on the Rivendell site. Velo Orange sells a 10mm spacer that's drilled and tapped for a bell.

The extra long threaded part scares me, as that means you're putting the quill expander on the threaded part in most applications. I suppose such forks are made with thicker walls to compensate for that?

You never put the expander on the threaded part of the steerer.

OK, I agree with everything you're saying about the threads and expander. But I'm trying to visualize the situation you described with 20mm of spacers and a standard quill stem. It seems like you'd be pretty close to having the expander in the thread zone.

I mentioned looking on the Rivendell site for examples of headsets with spacers. I don't even know if those pictures are still there, but if you look at the quill stems they sell, you'll see why they have no trouble reaching the non-threaded part of the steerer.

Then why would you need the spacers instead of just setting the stem higher? And how do you not put the expander in the threaded section of those forks with 7" of threading as mentioned by I_brad?

Because a quill stem that sticks up too far above the top nut looks lame and it's weaker than if it's supported by an extra long steel steerer.

I don't think I've ever seen a fork with 7" of threading. The replacement forks I've been looking at for an old Raleigh Competition have 50mm of threading. They'll work for a range of frame sizes. I didn't say they'd work for all sizes. I did see one with 100mm of threads, but that seems excessive.

If you look at the bikes on the Rivendell site, you'll see that most of them have headset spacers.

Ahh, good point.