subgrade

Hi all, so I am overhauling a commuter bike for my friend and it has a BB mounted chain case holder, thus also a K-type BB. My question is - if I don't find a K-type BB, is there any significant difference to a regular type BB (e.g. BB-UN-26 vs BB-UN-26-K)? I reckon the drive side cup on the K-type should differ slightly.

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.

CoachPerry19

While we are on the subject of bottom brackets, as a mechnaical newbbie, if I'm replacing a square drive crankset off of a 113mm spindle, am I looking for a replacement that specifically says 113 or will any square drive setup work. And if I understand this correctly my current Suntour POS has a chainline of 47.5 to 50?

Road Fan

I think that's not enough information to try to see what your chainline most likely should be. It depends on the purpose of the bike (road, mtb, commuter), double chainring, triple chainring, is the bike in original condition or have the wheels been replaced, what is the gearing in the rear, et cetera.

Are you buying a new crankset for the bike, or did you remove it for an adjustment or repair, or what? Is the BB trashed and useless? Usually if the BB is not worn out and it suits the crankset that will be on the bike, you don't have to replace it. Is the existing setup a double? Is the existing setup a cartridge BB or is it a spindle/ball/cup setup? If the latter, it's probably a good idea to give all those inner parts a good cleaning, at least so you can see if anything is worn and needs to be replaced. For many parts, most notably spindles, there may be numbers and alphabet soup that can be read off the cleaned part, that can be decoded to tell you exactly what you have, and give a lot of insight into choosing the best replacement or upgrade.

In general "just any square drive setup" might not work.

These days, old SunTour stuff is usually regarded as really nice old stuff. It will be worth it to put it back together correctly.

subgrade

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.

Wilfred Laurier

Is K type perhaps similar to E type bottom brackets? E type were made to accommodate a front derailleur mounted under the bottom bracket cup. I don't know if those derailleurs are still used...
Here's an example:
https://www.primeauvelo.com/en/bb-un...SABEgJaZvD_BwE

subgrade

Similar in theory, yes. Probably the E-type derailleur mount has a different thickness than a chain case mount, otherwise they could have just used the same code for both (or rather, no special code at all). Here's what a K-type looks like - I can't see no difference from the standard type.

https://www.internet-bikes.com/produ..._mm_117893.jpg

oliver1850

The K type is still available. Here's a 117.5 on ebay:

https://www.ebay.com/itm/New-Shimano...frcectupt=true

dabac

Then last outboard BBs I've bought have simply come with two spacers for the DS and one spacer for the NDS.
For a 73 mm BB shell, and no BB mounted accessory, use no NDS spacer and one DS spacer.
For a 73 mm BB shell, and a BB mounted accessory, use no NDS spacer and no DS spacer.
For a 68 mm BB shell, and no BB mounted accessory, use one NDS spacer and two DS spacers.
For a 68 mm BB shell, and a BB mounted accessory, use one NDS spacer and one DS spacer.
Does square taper do it differently? Haven't mounted one in awhile.