Hello,

I am looking to upgrade my fork on an old bike. What measurements do I need to take to find a fork that is compatible with my bike? I am looking for 1998 Trek y5. I am also interested in upgrading the rear shock and need tips for upgrading that as well. Any help is appreciated.

You need to match the steerer tube diameter and, if threaded, a near approximation to the original length or, if threadless, you will have to cut the steerer to the correct length. Also, you want to closely match the crown to axle length, that is the distance from the base of the steerer tube where the crown race sits to the center of the front dropouts. I'm pretty sure this is a suspension fork, so the a-c distance will have to be approximated by the compressed length with your weight on the bike or the fully extended length if the forks have the same amount of travel.

A google reveals some (old) threads on a mountain bike forum, further search there might help. Which Fork for my Trek Y5?- Mtbr.com
It gets dicey replacing 17 yr old ATB parts, as even NOS with shocks plastic seals age out.