Bottom bracket/cranks for 1x9 conversion

Hi All,

I have a Kona explosif from 1995 that I want/need to convert to 1x9. Need to convert because the left crank is fsck'ed -- the taper is going and the crank does not look like it wants to stay on. The current cranks are Shimano Deore XT, octalink, and the BB is (unsurprisingly) an octalink BB. This far the bike has triple at the front.

Because the crankset needs changing I thought of moving away from octalink and use something that will have more spare parts choice in the future. So, given that:

1\ the BB is a 68mm size
2\ I want a single ring at the front

what spindle length should I get (I might as well get a length that would allow me to mount the chainring outside and not inside the spider at this stage)? Also, I basically know shit about MTBs (I come from road stock) and I was wondering whether I should go for square taper, ISIS or hollowtech. I reckon that hollowtech would make my 'spindle length' question possibly moot, but I would not know how one controls spindle length in a hollowtech system.

Finally, I got a nice 41t chainring, 4 bolts, 104 BCD, so if you feel like recommending cranks compatible with the ring (and the BB!) do feel free -- I'm all ears.

Budget: I'd like no more than 100 quid, but I can stretch it to 120 ish.

Cheers

F

a cynical bump...

Slightly complicated one to answer this one because you have offered so many variables.

I'll start with the chainring you already have. As a 104BCD 4 arm ring you will be able to use pretty much any 4 arm crankset, whether square taper, Ocatlink, splined or HollowTech2 (external BB or HT2).
Shimano introduced 4 arm cranksets in 1997 but it wasn't until the following year that the 104BCD standard appeared. These were also square taper cranksets.

Octalink cranksets also used the 104BCD standard but this was relatively short lived between 1996 for Octalink V1, 1999 for Octalink V2 and the rise of external BBs in about 2006.
There are also splined crankset and BB interfaces too like Truvativ which also use 104BCD chainrings.
Most modern cranksets will also use 104BCD chainrings.

So a pretty large choice really.

With a 68mm BB shell you will also have a vast range of BB's to choose from.

In terms of BB spindle/axle length, as you say Hollowtech2 has a fixed length, so a shorter or longer BB is not a variable factor.

Most square taper cranksets (and Octalink) will have a recommended length which is based on chainline and chainstay clearance. It will vary depending on what crankset you are using. Many Shimano cranks which are 104BCD compatible use anywhere between 107mm - 118mm BB spindles.
For a good chainline you want to make sure that the chainring is between 47 and 50mm from the centre line of the seat-tube.
This will determine the correct BB length.

With an outside mounted single ring then you may need a slightly shorter BB to bring the chainring into a good chainline. However you also need to make sure you have at least 5mm of clearance between the chainstays and the ends of the crank arms.

With single ring set ups I tend to favour a chainring mounted inside the crank spider. It usually means you can stick with a standard length BB without compromising the chainline. This also works with HT2 cranks.

Once you have a crankset in mind then you can start thinking about what BB to go for though.

Alea iacta est... deore hollowtech crankset (three rings to ebay away) and super deal on a hope BB.