73mm cartridge bottom bracket in 68mm shell?

[ChrisEyles]

I have a 73mm octalink BB that I'm wondering if I can use in a 68mm shell frame.

I'm thinking if I use a 2.5mm spacer on the drive side the chain line would be the same (might see if a cassette spacer will fit). The only disadvantage I can see is that there'd be 2.5mm less threads engaged on the drive side. The left hand cup bottoms out against the BB cartridge rather than the frame, so a spacer should be unnecessary on this side.

Does this sound sensible? Happy to buy a 68mm BB if necessary, but might as well use the one I've got if possible.

 

[Ajax Bay]

PM me if you decide to procure a 68mm octalink BB - I have one (with some nice 105 cranks). What you propose sounds like a bodge too far and a hostage to fortune, but others can offer more authoritative comment/advice.

 

[biggs682]

Done the spacer trick quite a few times never had an issue .

have also seen it done on quite a few older 68mm cup and cone bottom brackets as well without any issue's ie my Aende had an issue with chain catching on fr mech cage and general poor gear selection so when imy lbs had a look the chainwheel was miles out from a good chain line due to a spacer being fitted from new on the drive side once removed all is good

 

[ChrisEyles]

Thanks for the replies. @Ajax Bay is that a road or MTB octalink BB you have (I believe the splines are different lengths on the two)?

 

[Ajax Bay]

Road: Shimano 105 5500 Octalink Bottom Bracket 68 x 109mm (and 5502 cranks)

 

[ChrisEyles]

Thanks @Ajax Bay - I need one for a Deore triple crankset, so would need the MTB model.

Anyhow, if the spacer bodge is good enough for @biggs682, it's good enough for me!

 

[Lookrider]

PM me if you decide to procure a 68mm octalink BB - I have one (with some nice 105 cranks). What you propose sounds like a bodge too far and a hostage to fortune, but others can offer more authoritative comment/advice.

Sorry for butting in abruptly to this old post
My search skills are poor and memory worse
But did you once say ( or involved in a thread) about wear factors on the NDS of BBs
Somewhere along the line it was mentioned in that thread that a guy had a lot of DS cups that were in ok condition
I am after a DS sram GXP english thread external cup ...
If you had nothing do with that discussion
Please accept my apologies...( I have tried online used bike parts websites)
Thank you

 

[Ajax Bay]

3. The bearings on square taper BBs with cartridge are big enough and last a very long time. It is the large hollow BSA BBs that are problematic, the ISIS and Octalink styles. These have a large OD and force the manufacturers to use undersized bearings with predictable results. As for stiffness, it is a function of the frame, not the BB. No matter how wide you make the BB, the flex is taken up and resisted by the frame in the downtube, seat tube and two chainstays. Ironically, the outboard bearings as in Shimano Hollowtech, Campag Ultratorque and SRAM GXP has a stiffness problem in the design itself. These bearings rely on the shoulder of the bearing cup to sit firmly, squarely and evenly against the edge of the BB shell. This requires a very precisely reamed BB shell, so precise that even paint can spoil the face and lead to premature bearing wear. What happens here is that the cup moves on its own threads with pedal cycles. This causes the bearing balls to run out of alignment with their races and gall. Still on that topic, the flex in the hollow shaft on the left side of the crank exacerbates this out-of-line running of the bearing and left cups fail long before right cups. To the casual eye it may seems that both sides receive the same forces but they don't. The left end of the axle is subject to torsion whilst the right side not at all.

Sorry @Lookrider I only have one bike with GXP and have not had issues, but doesn't get ridden much. Can't recall the thread you half remember. I'd put an ad in the 'Wanted' sub-forum.