I'm sorry but I'm not really following what "bolt circle diameter" is......... Oh OK I found out what it is,
BCD
I'm pretty much limited in changing the cassette to something more lower gears. It's a 12-28 and I don't want to start with messing with MB parts. What I have now is pretty close to what I want. If I did want to move in any direction, it would be to getting lower gears. If I had to change crank, cassette, derailleur and who knows what else, I'd just live with what I got. I just thought I could change the rings, or at least think about it.