here is a link to a page that might help.http://www.tractorparts.com/spec_sheet.htm
Would be helpful if we knew which tractor your working on.Cat never seemed to put that in there manuals,but do think I have seen a link to some one that has put the numbers together.I will say I have used 2 or 3 more plates than a new steering clutch would us when using used parts.Even though the manual says to us only one.Since they were the metallic ones they never gave any problem.So it depends how hard or how much you intend to use this tractor,and if you can find some extra used parts to bring the thickness up. Ray