[chruby99]

Hey guys I am rebuilding my old school Gary Fisher. I am installing a XTR M952 Crank set. I don't know what size BB to buy. I am pretty sure it is a 68 mm cup (my XT M760 External bearing cups thread onto the frame) I don't know what width to go.My BB tube (part of the frame) is 67.5 mm wide or roughly 2.65625 wide. From what I have seen the M952 BB are 68mm and 73mm dependint on the spacers.

Can anybody give me an Idea of what size BB to look for.

 

[Bikinfoolferlife]

The 112.5 spindle is used to achieve a 47.5 mm chainline with either a 68 or 73mm width bb shell, the 116 to achieve 50 mm chainline with a 68 or 73 mm shell (what you call the tube), using spacers depending which width shell you have.

Your external bb cups will thread onto a 68 or 73 mm shell btw, you use spacers to set up one width over the other. The diameter of bb shell is different than width, and that's 1.37 inches (35 mm).

 

[chruby99]

Ahh

I guess I never put 2 n 2 together if the shell is 67.5mm wide, the diameter of 68 or 73 would be BMX-ish. My wheel is running a 46.037 mm chainline so I guess I am looking for a 112mm wide BB. Thank you for the enlightening information.

 

[Bikinfoolferlife]

Didn't realize the bmx bb was such a different dimension. How'd you measure the chainline for your wheel?

 

[chruby99]

A bmx shell is 2 in.in diameter.
I measured my chain line by measuring the axle width (5 3/8 " or 135mm) than the distance from nondrive drop out to the center of the cog (4 1/2" or 114mm) I subtract half the axle width from the cog measurement 135/2=67.5mm 114-67.5=45.5mm or 46.037 if I use exact standard to metric conversions for the math. Basicly my measurement is the distance the cog is from the center of the axle or the center of the frame if the frame is strait.

 

[Bikinfoolferlife]

With such a precise number I was curious...easier to just keep all measurements in metric. What kind of chainring is on that xtr crank? Does its position equate to the middle or large ring?

 

[chruby99]

Good question

It is a boone ti ring. I never had a spider mounted on this crank set so I dont know. I do notice that XTR spiders are dished and this boone ring is flat so I am hoping that it is equivelant to the middle chian ring.