collector assembly.

Ok i had an issue. The ones that came with the hedman collectors the bolt circle isnt big enough. None of the holes line up with the bolt pattern on the collector itself. The inlet is the right size. So I went and got normal 3 bolt ones from advanced and they too are not a big enough pattern. Im not sure of the brand of collectors since they didn't come with the headers used when i bought them. Were there different sizes? What is the correct method of measuring them?