Bottom Bracket question: would a 68 x 117.5mm work instead of 68 x 116?

ThreeBlindMice

Would I get away with fitting a 68 x 117.5mm bottom bracket instead of a 68 x 116?

Its off an old MTB using a Shimano Altus BB supported front mech.

I cannot seem to be able to source a 116 mm BB anywhere other than across the Atlantic and at that its not worth it.

Paul Kiernan

It shouldn't make any difference. The cranks will 1.5mm further apart and the chainline will be 0.75mm further out than it was previously but that's not enough to matter.

ThreeBlindMice

Paul Kiernan wrote: »

It shouldn't make any difference. The cranks will 1.5mm further apart and the chainline will be 0.75mm further out than it was previously but that's not enough to matter.

Cheers.

As long as it doesn't effect the front mechanism back plate.

Holyboy

If you are planning on using you currant BB fitting mech or E type as they are called you really should use an E type BB for two reasons,

1/ The spindle on an E type BB is adjusted 2.5mm to the non drive side to allow for the extra space the mech adds to the drive side so if you use a standard BB your chainline is going to be another 2.5mm out on top of the extra axle length.

2/ The body of the BB is also altered so as the non drive side cup will make contact when fitted, if you use a standard 68mm bodied BB and the non drive side cup has a flange on the outside of it like many do, when the flange stops at the frame the cup will be about 2mm short of where it should be and won't support the rest of the BB properly.

E type square taper BBs are getting fairly hard to get these days.

Now if you can get a 68mm bodied BB that has a non drive side cup without a flange you could get away with using it but the non drive side cup will sit inside the BB shell a bit and instead of a longer axle you would need slightly shorter by about 2.5mm but I would imagine a 113mm spindle would be fine.

But depending how old it all is, it could be the old micro drive system which used a 38 tooth large chainring, if this is the case and your bike can be fitted with a band on mech I would suggest dumping the front mech and replacing it with something that can be moved up when your chainrings wear out and you need to replace the chainset, the closest reasonably priced chainset you will get to a 38 tooth is a 42 tooth and your currant front mech probably won't work unless it has two sets of bolt holes so you can raise it up but I don't ever remember any of the mico drive mechs having this, it is a long time since I've worked on a bike with one though.

If it's not micro drive then disregard the whole last bit obviously.

Phew....that took ages......how many times did I type BB......too many probably!!!!!

Eamonnator

9

Paul Kiernan

Eamonnator wrote: »

9

11 actually, if you include the rhetorical question!

Eamonnator

Paul Kiernan wrote: »

11 actually, if you include the rhetorical question!

You must have as little to do as me.but obviously you can count better.