Installing HollowTech II Bottom Brackets
The new style external bearing bottom brackets are challenging to work with at times. Take the time to measure each part of the assembly to make sure that everything will go together as planned.
50mm or 47.5mm Chainline
Shimano documentation says that all of the HollowTech II MTB cranksets (except DXR and Saint 150) produce a 50mm chainline. Interestingly, after measuring and playing with the parts, a 47.5mm chainline is possible by removing one of the spacers on the drive side. This is not important to people running 3 rings up front who will want the 50mm chainline, but for people runing 2X9, 1X6, or 1X1 configurations it is very good news. An added benifit is that the pedal positions actually center better when set up at 47.5mm.
Sometimes the Bottom Bracket Shell is Too Narrow
While this example is for a bottom bracket shell that is less than 68mm, the same solution is true for one that is less than 70.5mm or 73mm.
Please excuse the dirt and filth on the parts in this discussion. I was in a ghetto mood, but the result of the work was clean enough for great results.
Measuring the bottom bracket shows that it is well made and will install perfectly for a bottom bracket shell width of just over 68mm with 2x 2.5mm shims.
The two shims. They will be mounted on each side of the bottom bracket shell to space the pedals evenly and attain the final chain line, 50mm.
The bottom bracket shell is 67mm, 1mm short of optimum 68mm. This is a result of the amount of matirial necessaryly removed to properly face the shell.
A 1mm shim needed to be installed on the drive side in addition to the stock spacer to correct for the short bottom bracket shell. Shimano instructions would have me place 5mm of spacer in this postion, but the 3.5mm spacing I added will keep my chainline more inboard for my 48.5mm chainline Sixxer.
Just 1 2.5mm shim on the non-drive side.
The crank installed with proper 68mm shell spacing, 50mm chainline, and crank ends centered about 13mm from the chainstays. The crank should spin freely.
Comercially Available Spacers
Almost all bottom bracket and cassete spacers share the same convention of having a 1.375" ID. The OD may vary, but having a range of adjustment in thickness is what is important.
Shimano shows that it provides these spacer sizes:
- 2.5mm (#Y-1F8 13000)
- 1.0mm (Dura-Ace 10 Speed Cassette Low Spacer #Y-1Z8 07000)
Other companies may offer:
Service Instruction for XT Front Drive System
Shimano Service Instruction for HOLLOWTECH II