Looking to buy a new fork for my bike and need to know what I me to know when purchasing one that will fit. I have a campy headset and it is a threaded fork...I think the bike is 53cm and it's a Pinarello

You need to know more than that.

yeah, like headtube length and headset stack height
(campy track hs stackheights can be anywhere from
around 34mm to 39mm)

And make sure you know whether it's 1" or 1 1/8". (Outer diameter of the steerer tube). Both exist AFAIK. Apparently, there is also some variation in the internal diameter, which determines what stem you can use with it.

I guess you can just take a tape measure to your current fork to see how long a steerer you'll need. (assuming that frame and HS will be the same). A longer steerer can be cut to fit, I presume (?), so just make sure it's not too short. Then a caliper tells you the other 2 sizes.

SO basically take it to the shop

i would. take it and make sure the dimensions are the same. when i replaced my fork, i had my old one for comparison, and i think it helped a lot.

how about the inches of threaded and the inches where it isn't threaded. THe one I am currently look at is 6 3/8 inches in length, 2 1/4 inches of thread

just measuring my headtube...it looks to be 5-3/4inches without taking the fork out...seems like the one I'm looking at will definately be too big huh?