You may want to consider replacing the nuts while you're at it. If there's any wear or, particularly, corrosion, the threads will have lost much of their retaining capability.
Incidentally, an 18" Crescent spanner will be much stronger than the correspondingly-sized open-ended spanner; don't worry about it opening up. FastBottoms is in the business; his recommendation for a pipe wrench (wouldn't know your terminology -- what's the British term for pipe? I believe we have spanner understood) would be well-founded and based on his personal experience.
Last edited by jones2r; 01-16-2007 at 11:48 AM.