Do you have your old fork? Has it been removed from the bike?

If so measure the portion which runs through the frame and has threads on the top, this is the steerer tube. The fork crown is the top which holds the two legs of the fork together and joins to the steerer tube.

If you check out this listing you will see a good example of a fully exposed chrome fork.

http://cgi.ebay.com/Chrome-Marinoni-...3%3A1|294%3A50

I believe this is listed by our own Lynn Travers, aka CV-6.