I want to to replace the OEM 63mm suspension fork on my Trek 820 with a rigid fork but the two cheep ones are rated to be correct for 85mm. Nashbar sells a carbon fiber fork for a hundred fifty bucks that is for" forks with less than 80mm of travel." but for less than the cost I can get a Rock Shox Dart 2. The carbon fork o rht e Dart would cost TWICE what I paid for the bike, so I'm hoping some one will say that i can replace with the 85mm correct fork.

You measuring travel distance .. ? .. fork crown race to axle is another way to measure replacement forks

OEM get pallets of Boxes of parts , buying parts aftermarket , retail, in the US will cost more.

unless you travel to Taipei or Hong Kong, and buy stuff there.

closer to where it all is made.. .

All info from the words on the websites. No actual measurements were made on anything. Measure fully extended? As it sits? Rider on board?

just look to measure your Axle to crown measurement on your existing fork and look for something close - a little bit taller will be OK. If you go way too tall you will slacken your existing head angle and your steering will feel a bit sloppy. I personally like a slacker head angle - my first MTB had a 72* head angle (quick steering). Now My bike has 69* head angle (more relaxed steering)