I found a threaded fork to match my frame, but it is ridiculously long (240mm) whereas I really only need 155mm or so (54cm frame).

Scuttlebut is IF there's enough steerer tube left after lopping off the threaded portion, there's no issue installing this as a threadless fork. Thus, if fork steerer is 240mm and I need only about 215 mm worth of steerer (at MAXIMUM), then is only a question of if the threads extend much past the 25 mm cut off point. I estimate about 3mm worth of thread could remain after cutting to correct length. Is that still too much thread to clamp a threadless stem to (using a shim, of course)?

That's pretty much right. As long as the threadless stem can clamp onto the steerer below where the threads were you are ok. You do not want to clamp the stem over any threaded section.

Yeah, looks like about approximately 3 mm worth of thread will remain after cutting.

Rather than risk cutting it short you might consider leaving it a little long and adding some small spacers above the stem. I don't think there is any problem having spacers along the threaded area. You can make minor stem height adjustments by shuffling spacers around. You can always shorten the steerer later.

Your tube should be threaded 24 threads per inch. If you convert your 3 mm to inches you have less than 1/8 inch. So you would have less than 3 threads on the tube. I think you can safely ignore the threads and use the tube as if there were no threads. With only 3 threads the tube is not going to collapse if you clamp down on it.

Now if I were doing this, I would leave enough thread above the stem to to use the original nut to put the preload on the bearing. Not that it would work any better than a star nut, but it would make it a head scratcher for any bike cognoscenti that happen to spot it. Plastic caps are available in the right size to cover the hole.

Now THAT's an IDEA! But it would probably work it's way loose much quicker than anyone would like, I imagine.

Wouldn't matter. Once the preload is set, the stem is clamped to the steer tube and holds it.

Hah, cool idea! Easy to prevent it loosening: tighten it a bit more after clamping the stem onto the steerer.

Have a question, are u using spacers?? if you are then take one spacer out and chop chop the threads, second solution is to find a stem where the clamp are is shorter and chop chop again?

Doubt you will have issues as you are right now because like 90+% of the stem will be well clamped in the non threaded area.

Sure, or there's always good ol' loctite.

Ha, thats awesome. I've never had the luck of landing in this sweet spot when using the unthreaded steerer threadless. If I do, I'm for sure doing this.

Hey, I need to do something similar: need advice on best way to do straight cuts.

Do you have access to a vice with soft jaws? Or one of those Workmate things? Ideally you need something to reliably hold the forks still.

Plenty of those around. You can use a hose / jubilee clamp. You can take a fairly non-stretchy tape and wrap it a couple of turns. If you get it to align edge-to-edge it'll be a straight line.

Or you can buy one of those purpose made saw guides. Or a dirt cheap miter box from any DIY store.

Thank you, and Airburst.

How about those pipe cutter tools? They only seem to cut copper, and I am not sure if they are made to the needed diameter for a fork, but I was toying with the idea of buying one and try to see if I can at least mark the circumference precisely with it.

We used to use one at the co-op I volunteer at, whenever we cut forks and handlebars down, but we went through an awful lot of cutting wheels for it. They also, in my experience, have a tendency to move sideways across the surface rather than cut into the steel, since steel's much harder than copper. Ours was size-adjustable though.

We now have a Park Tools cutting guide, but in all honesty, I normally just put the soft jaws in the vice, stick the fork in it and cut it by eye. I can get a fairly straight cut by doing that. I also file a taper onto the top of the tube, rather than leaving it flat. I find the stem goes on more easily that way.

I'd heard that the pipe cutting tools can also deform the fork a little bit. No?

Pipe cutters are a plumber's tool; they don't belong anywhere near a bike.

My advice for someone least equipped to do it: wrap some tape around the top of the bit you want to keep, nice and straight. If you don't have a vice, it'll be a lot easier than a hacksaw if you have an angle grinder with a cutting disk (don't even think about it if it's a 9"), but either way you need to find some way of securing it pretty well while you cut. Start your cut next to the tape, and simply cut away from it a little, ensuring that if you cut off more than you mean to it's no big deal. If you're using a grinder it's easy to square it up now (hold the tool down as an impromptu bench grinder; a fork is light enough to grind with one hand; 4mm cutting disks can take quite a bit of grinding on their side before they become dangerous), otherwise you have to file it down. Put a chamfer around the top, down to the depth of the threads, and use a bit of extra care screwing the cup on the first time.

If you're cutting aluminium, it'll make a bit of a mess, but you can clean the steerer up with a file and the disk with a rock. If you're cutting carbon, forget using a hacksaw or a file (delamination risk), and use a good mask.

pipe and tubing cutters are different but they are a poor choice to cutting steerers. i don't even like using them on handlebars

OK, then soft jaws in vice it is. I definitely plan on filing a chamfer onto the rim of the tube.

Dammit, chamfer! THAT's the word I was searching for!

Yes they do. How much depends - as usual - on a combination of material, skill and tool quality.
Some people report that they get an externally raised area, but I can't say I've ever been troubled by that.
I do get an internally raised lip though, so the slick thing is to mount the starnut first, before the cut.
I like using it, but it's not like I'm not stumped w/out the pipe cutter. Cuts get straight and burr free.

They come in all shapes and sizes, and in several quality levels as well. Mine will do steel w/o hesitation, and I'm rather fond of using them.