68mm BB in a 73mm shell ?

RedDev21

Hi all, is it ok to fit a 68mm bottom bracket (Octalink splined) into a 73mm BB shell? I originally had a 73mm BB in the shell but the spline was too short which made the small chainring touch up against the side of the frame. I had a spare 68mm BB with a slighly longer spline so I fitted this and it cured the problem of the chainring rubbing on the shell. There doesn't seem to be any obvious problems, nothing is loose. However, as Im new to this bike building, just wondered if it will cause any problems?
Also, when I swapped BB's over, I just removed the drive side and replaced that, leaving the old cup in the non-drive side as the cup is threaded and couldnt get it out. Both BB's were Shimano sealed cartridge units, just slightly different sizes. (68mm x 118) & (73mm x 113).
Hope this isnt too confusing! :?

nicklouse

not really as the bearing on the non drive side will not be properly supported. also you may find that the non drive arm catches on the cup.

BUT as the axel is over length and it works then the only consideration is the lack of support of the BB.

RedDev21

So is there any way I can put the correct BB size back in the shell and stop the small chainring rubbing against the frame?

hoathy

The lack of support for the non-drive side is really bad though, as all of the stress is nowing to be going through the driveside thread with more leverage than it shoud ever take. I think running it like this for any length of time will lead to a messed up drive side thread. If it were me, i'd go and buy a new bottom bracket. asap.

nicklouse

yes get the correct 73mm bb with the correct axle length.

Frame? cranks? year?

RedDev21

Im building up an old Trek 4100 frame (2003) for a commuting bike for work as I dont want to do 20 miles a day on the road on the full susser. Beginning to think it would've been alot less hassle and cheaper to have just bought a new bike but Im determined to finish it now! The cranks are hollowtech1 octalink deore + deore front mech.
Thanx for your help !

nicklouse

what year deore? got a picture.

RedDev21

Originally taken off 2008 Specialized FSR expert. Have upgraded the Spesh and am using its parts on the new build.

nicklouse

are you sure the trek is a 73mm bb? i thought they were normally 68?

hoathy

I have a 4500 for 2006, thats a 73mm, so its definately possible.

RedDev21

Just measured the width of the shell with ruler, it is 73mm

nicklouse

just that i have to say i have never noticed that the treks i have built were 73mm. But i was not looking

there reason for the ask about the year was that some year cranks on the HT1 had some really odd axle lengths... upto 127 axles...

if you measure what the current chain line is at the front with the 73mm bb then that will give you an idea as to the axle length needed on a 68mm bb.

skyliner

I don't think there'll be a support issue on the cartridge, as it has the non drive cup from the 73mm version fitted. The cart is the same length on both BB's and the non drive cups are made longer on 73mm variants to accomodate this.
My concern is that the chainline will be out by about 2.5mm to the drive side because of the axle position on a 68mm BB. and as Nick said, you may get rub on the crank/BB cup, or heel rub on the chainstay on the non driveside.
If it's an OEM Spesh Octalink chainset, it should run a 118mm axle with the correct shell width (73mm on the Trek) to get perfect alignment.