Originally Posted by
ljsense
I don't know what a k-type bottom bracket is, other than it is seemingly intended to have a chainguard between the fixed cup and the frame, but since no one else has weighed in, I'll share my experiences using standard square bottom brackets on bikes that have a chain guard.
The chainguard is about 1mm or less in thickness. When you tighten the fixed cup with the chainguard behind it, it torques the chainguard with it. So maybe a K-type has some modification to lessen or eliminate this effect. But once you figure out how to get the fixed cup tightened and the chainguard aligned right with the frame, you're good. If the spindle length is the same, the chainguard is not going to throw the chainline off in any meaningful way.
You're right, the K-type is just that - a BB intended to have a chainguard mount between the fixed cup and the frame. I don't see any modifications on the old one as to hold the chainguard mount in place while tightening, but I don't think that's going to be a big problem. I was just wondering if the thickness of the chainguard mount does affect the width between cups to any noticeable extent, and therefore a 68 mm K-type would be in fact, say, 69 mm, to compensate for that. Your answer seems to indicate that there in fact is no difference, and I certainly can't spot any when looking on online pictures. I would measure a standard BB of the same size if had one lying around, but I don't.
This then leaves me wondering what was the purpose behind the different model code, if there isn't any real difference? It's not like the K-type is more expensive or anything.