I have a head tube that is about 4.25" long. I also have a fork that has a neck that is about 6.75" long. The threading is 1.5" long.

6.75"
-4.25"
2.50"


2.50"
-1.50"
1"

Assuming that I'm doing the right math, would I be able to put on 1" of spacers on a threaded fork/headset?

Sure , It would be between the top locknut, and the top headset race.

In other words, the spacers would go between the two threaded parts. This means the fork steerer needs to be threaded far enough down for the threads to reach the upper headset cup (or upper race if so configured).

If it's not , then one of Sheldon's kludges can be brought in,
threadless top race, a clamp spacer, with 1"ID[25.4mm] to hold the adjustment,
the spacer and the top nut.
then you can use your quill stem.

2.25"
-1.50"
.75"

what is the 1.5" is that the HS height? it may be close on the threads but ti should work. I seldom cut forks to exact demension. this leaves you room for a taller headset, a cable hanger, bag support, bell. you never know.

I have used longer stemmed forks by grinding out the threads of the bearing cap. I grind out just enough that
it will slide on the stem. Then I put a spacer on and use the top cap to tighten it all up.

The answer depends on the length of thread, and how far you plan to insert the stem.

Most headsets require close to 3/4" of thread, so you're at the borderline, and first need to test that the top bearing threads down far enough. If it does, then you can use extra spacers above, or trim off the excess at your option (don't trim too close, since you can't re-trim it longer, cut long and use spacers to be safe).

The second part is critical, the stem must extend 1/2" beyond the lowest thread if it's the split with cone type, or much more than that if it has a tapered wedge. That's because the threaded section of the steerer has less than half the strength of the unthreaded section. The stem must reach well into the full wall section to transfer the load without breaking the steerer.

This is what I was thinking.

What is a clamp spacer with 1" ID?

Thank you.

I'd want to use a second locknut if I ground out the threads on the top bearing race. The top race doesn't need to be threaded but you need two nuts to keep it all from loosening up when the steerer turns back and forth. Just putting one top nut on will eventually work loose as the bike steers.

What kind of spacers should I use and where do I get them?

Something like this (except in a 1 inch version):

https://www.universalcycles.com/shopp...5&category=100

This locking headset spacer is meant for threadless applications. With a threadless headset, normally the stem does double duty by also keeping preload on the headset bearings. The locking spacer allows the stem to be changed and/or adjusted without losing the headset adjustment.

If you can find a 1 inch version of this spacer, you could convert to a threadless configuration while still using your quill stem.

Cool. Thank you!

I can't find any 1" clamp spacers, but I did find a 30mm carbon spacer on Amazon for $8.

Will this suffice?
Should I go ahead and order it or go to a LBS?

I reviewed this thread and just noticed that you never told us whether you have a 1" or 1 1/8" OD steerer on this fork.

About plain spacers: It is quite OK to combine several spacers to get exactly the height needed.

Oops.
1"