Hey all,
I’ve got a FC-6750 HOLLOWTECHII Compact Crankset installed with a SM-BB6600 bottom bracket. When I installed the crank, I noticed that there was no way tighten the crank arms toward the centerline of the bike. It appears that the best one can do is to hand push the crank arms together and tighten the crank arm bolts. Am I missing something? Should there be a bolt that draws the crank arms together? How do the hollowtech cranks stay tight to the BB face?
Note, I haven’t had a problem with this crankset, I’m just curious.
Hey all,
I’ve got a FC-6750 HOLLOWTECHII Compact Crankset installed with a SM-BB6600 bottom bracket. When I installed the crank, I noticed that there was no way tighten the crank arms toward the centerline of the bike. It appears that the best one can do is to hand push the crank arms together and tighten the crank arm bolts. Am I missing something? Should there be a bolt that draws the crank arms together? How do the hollowtech cranks stay tight to the BB face?
Note, I haven’t had a problem with this crankset, I’m just curious.
I just did this installation last night following the park tools website directions. They only mention tapping the cranks with a mallet to make sure they are pushed all the way in. There is not a bolt that draws everything together with the Hollowtech II platform. I think if there was, people would overtighten the crankarms together and squeeze the external bearings too tightly.
There’s a cap screw that threads into the end of the spindle on the non-drive side that tightens the 2 crankarms laterally against the BB cups. It has a recessed ‘star’-shaped head on it that mates with a special plastic tool.
Edit - here, found the tool… the recessed star is pretty shallow and the tool is a plastic dial as opposed to a wrench-type lever, to help avoid over-tightening:
The cap that goes on the NDS helps bring the left crank arm toward the bearing cup. This doesn’t draw the two crank arms closer together. The only way to bring the right crank arm toward the bearings is to push it in or tap it with a rubber mallet.
Note: I’m not trying to be disagreeable; I’m just trying to help clarify for the OP what the purpose of the cap is. Mentioning the cap is a good point and one that I forgot to talk about in my earlier post.
How do you figure? Unless it’s dramatically different than the 6600-series crankarm/spindle interface (I realize the big ring design is quite different on the newer sets) the lip on the cap seats against the rim of the recess on the NDS crankarm, while the threads go into the spindle–so that pulls the crankarm against the driveside crank affixed to the opposing end of the spindle to form a ‘sandwich’ with the BB cups/shell. How could it pull the LH crank towards the cup (as you describe) otherwise, if it’s not pulling against the integrated spindle/spider assembly on the RH side? What else can it be tightening against? I can’t imagine the Shimano engineers would have botched something that basic going from the 6600 to the 6700-series.
My understanding is that if you don’t have the right arm all the way in, this cap is not going to pull it up against the bearings on the drive side for you. I’m basing this off of what the Park Tool website says on this installation (found here).
Specifically, they use these two phrases when discussing the cap:
1: “A crank cap is used on the left arm for bearing adjustment. The cap does not hold the crank, it is only for adjustment.”
- “This cap acts only to push arm fully over to cup bearing. This cap does not tighten the arm onto the spindle. IMPORTANT NOTE: Secure cap gently. Use the eight sided stud at the end of the BBT-9 and tighten only 4 to 6 inch-pounds.”
4-6 inch-pounds is practically nothing. That can’t be enough to truly pull everything together, particularly if the crank arm and spindle have caught on something. This is why I said it is the installer’s job to make sure the right crank arm is fully pushed against the bearing, using a rubber mallet if necessary.