Bicycle Mechanics - How to go from threadless to threaded?

I have a GT Outpost, AL frame, 1 1/8 steerer, but I am not sure if it's threaded or threadless.
Can someone tell me how to determine which it is?

Anyway, I am looking for suspension fork for this bike, and depending on what I find on craigslist or ebay, I might need to go threaded or threadless.

What I am asking is, how hard is is change from threaded to threadless or vice versa?

TIA

your fork is gonna be threadless

is there a nut on your steer tube underneath your stem? Most likely your bike is already threadless as well.

If you have a treadless bike odds are you cannot go to a threaded fork. 98% of threaded forks are 1 inch and 98% of threadless forks are 1 1/8. The remaining 2% from either catagory are too hard to find to even consider.

Yes, there is a nut under the stem. The stem on the GT looks basically the same as my Raleigh SC30, which is a threaded steerer, or so the Raleigh specs say.

They both look exactly the same, externally anyway.

Like Cynical says... you're gonna have a tough time finding a 1.125 threaded fork.

You could always convert it, but that would mean in addition to your fork, you'd need a new stem and headset.

Sometimes one measurement is worth a thousand guesses. Take off whatever you have to remove and measure your fork's steerer tube outer diameter. If you don't have a caliper, snug a crescent wrench around it and measure the distance between the jaws. Since the only logical choices are 1 inch and 1 1/8 inches, you don't have to be super accurate.

If it's really 1 1/8", you're golden. In addition to your suspension fork you need to buy a new threadless headset and a new stem. If it's 1", you can probably find a suspension fork that will fit it but don't expect much from it.

Ah, I think the light is beginning to shine through for me.

So basically, my GT Outpost almost surely uses a threadless steerer, but my Raleigh SC30 is some kind of anachronism in that it uses a threaded steerer.
So, if I ever want to change out the RST 781-CL suspension fork I will have to also change to a threadless headset?

I'll try to take off the forks of the GT Outpost and measure them, but having never removed forks before, it might take awhile. I'm sure it's an 1 1/8", as I have an older GT Outpost cro-moly frame that is definitely 1".
Basically just a parts bike, but it sure did ride great.

Trust me, you don't want to buy a threaded suspension fork.
Threaded stuff is completely obsolete now except on the lowest of low end bikes (with few exceptions like some track bikes). Any threaded suspension fork you find will more than likely be old and beaten to death by the previous owner or crappy to begin with, probably both.

Most likely, your headset, stem and fork are t-less anyway. Google image search "threadless stem" and "quill stem" and you'll see which you have.

If you have a treadless bike odds are you cannot go to a threaded fork. 98% of threaded forks are 1 inch and 98% of threadless forks are 1 1/8. The remaining 2% from either catagory are too hard to find to even consider.
1" threadless fork (road) is not hard to find at all. I have one and know about many others. Finding a HS was very easy, too. You're probably right about the 1 1/8 threaded, though.