Yes, there are so many 3-letter words out there that the vBulletin software upon which the forum is based sets a 4-letter word limit otherwise you would be innundated with results in many cases.
Another way is to use Google and enter:
"your topic" site:http://www.picaxeforum.co.uk
Might also be better to have "i2c" and "some other keyword" to help fine tune your search and reduce the quantity of results you receive