I can find threads all day long, but I want to find a particular post inside of a thread that has 140 pages in it and I can't get the search to go lower than the thread level.
The way I do it is to go into the thread, then click the main search button at the top of the page. It should drop a pop down (so it doesn't take you off of the thread). One of the boxes you can check is "Search this thread only".