Most threaded replacement forks have about 2" of thread. A typical headset and spacers needs something less than 1" or thread. That means you can buy a replacement fork slightly taller than yours and trim it without needing to extend the thread (which adds new problems0.
The dimensions you need to be concerned with are total length which has to be longer than yours, and the thread length. The unthreaded length (overall - thread length) has to equal your old fork's overall length less one to two inches.
BTW- many replacement forks are sold with tall steerers, with 4-5" of thread. This excess thread length can be a problem, (unless you're cutting it off anyway) because if you have more than 2" of thread the stem might not extend into the unthreaded area, increasing the risk of steerer breakage over time.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
Last edited by FBinNY; 01-10-11 at 10:18 PM.