I tried this method and I also got a couple duplicates of the same thread. Example, search 'siebenrock' both ways. Site internally you get 7 hits with each occurrence of the search term highlighted. Google using this technique, you get 10 hits, three of which are dupes.
When you click the Google link, sometimes you get just an unformatted view that shows only the post containing the search term with not much context. To see the context, you have find the 'Go to full version' link at the bottom which takes you to the top of the motobrick.com thread. Even if you add quotes to the Google search term, you get hits that are out of context and not useful.
Some hits are not found on the search engine level but are found using the Search box. For example, search 'pixie'.
When I used the site's search box I got the full view of the entire thread, in which I could then just search for the term on the page using the browser's Find command.
So not to be a contrarian but I'm not sure what the advantage of this is over the site's Search box. If I want to find motobrick hits I just use the Search box. If I want to go outside, I do a web search. Not sure what using a search engine to restrict the search to a site that has a functional search engine brings. Laitch, have you found hits on the search engine level using this technique that aren't found using the site's Search box?