[Lynne]

Take a look at the funciton fetch_similar_threads. There is a hook in there that you can use to hook into the query - search_similarthreads_fulltext

You should also note that similar threads are determined at the time the thread is created - they are never changed unless you run the similar threads update in update counters. So, an older thread will only have even older threads listed as similar threads (unless you update them in update counters, as I mentioned).