my 68mm bottom bracket shell

hummie said:

ive an unknown steel frame.

im trying to figure if i can get a wider or narrower isis bottom bracket and adjust the width from the center of the bike. can i just do as long as the chainrings dont rub the frame? the goal being get as close as possible but not touching?

i see a lot of lengths. ISIS Bicycle Bottom Brackets - Modern Bike
will i have to wait till i get the frame and put sprockets on to know what size will fit?

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There are two size measurements to consider for an ISIS cartridge bb. I assume you're looking for this type to use with an old ISIS crankset?

One measurements is for the shell width so you need a 68mm version, simple enough. The other measurement is for spindle length, not so easy.

As mentioned finding the correct spindle length is going to be very difficult as you'd just normally replace an existing one like for like using the measurements and bb standard (e.g: BSA/1.37"x24) printed on it along with the correct spindle length specific to your bike.

Some very careful measurements may help finding what you need but even then it's going to be tricky to predict final chainring/s position.

You're not aiming to get your chainring/s as close to the frame as possible you're aiming for a suitable chainline for your frame and drivetrain set up. Somewhere around a 49mm chainline would generally be a safe bet for frame clearance and decent drivetrain performance.

Having said all that and as mentioned you're probably much better off changing to an external bb set up with a new crankset and throw out the ISIS crankset if that's the case?

im getting a frame made. it's going to be 73mm bottom bracket shell and it was a miscommunication between them and I.

im running it single speed and want to get the chainring as close to the chain stay as possible without hitting. i think i'll have to wait till the frame comes to see what will fit.

i assume isis isnt going anywhere and maybe things are slowed these days but all the sizes will be back in stock. right?

im thinking of brazing a nut on the bottom bracket of the frame that will take this:
Mr Control SSP-11-1 Single Speed Chain Tensioner - The Bikesmiths
instead of running it on the derailleur hanger.
like this:
YESS ERT-B Tensioner Bottom Bracket Mount | Time 2 Shine BMX
if you have any thoughts. using a 50 tooth cog and 17 tooth chainring so...mabye you can guess whats going on here. Cranks - Freewheel ISIS Crank Set - Sick Bike Parts

and this is going to be part of the seat tube

belt down to the double freewheel on the crankset and standard mtb singlespeed rear end (except the tensioner at the front)

ISIS BBs consistently gave trouble (creaking, bearing failures) as the larger diameter spindle didn't leave enough space for decent bearings. (Octalink did better with this, but Shimano never licensed the BBs and are the only source for them.)

The only possibly decent ISIS BB is SKF, available only from Rene Herse. These have different bearing construction and a long guarantee.

Re sizes: ISIS BBs came in 3 sizes:

1. 108mm, road double, 68mm shell
2. 113mm, MTB triple, 68/73mm shell (adjusted with spacers)
3. 118mm, road triple, 68mm shell

AFAIK, nobody made any other sizes, ever, except perhaps one or two for fatbike 100mm BB shells.

ISIS has been obsolete/failed technology for around 15 years now - at least! I'd stay well away; both square taper and external bearings are more reliable and available - as is Octalink, but that's obsolete too and on it's way out. One day Shimano will stop making Octalink BBs, and then everyone with them will be screwed.