Bottom Bracket Service: BB90, BB86, BB92, GXP® Press Fit
This article will review the service of the SRAM® GXP® press fit and "Shimano® Press Fit" threadless bottom bracket standards. These systems use a two-piece crank with a nominally 24mm diameter spindle.
The “Shimano® press fit” standard is a bottom bracket system that does not use internal threading. Unfortunately there is no industry wide agreed upon name for this standard. The Shimano® Company’s name for the system is “Shimano® Press Fit”. The colloquial titles BB86, BB90 or BB92 refer to the nominal shell width, some with and some without pressed bearings. The pressed cups have a lip of 2mm, which widens the shell from 86mm to 90mm, hence the name BB90. The two-piece cranksets are typically the same ones used in the “external bearing systems”. Cranksets the Shimano® Hollow Tech II, FSA® MegaExo, Race Face® X-type and Truvativ® GXP® are used in this design.
NOTE: It is the intent of the bike manufacturers that the bottom bracket bearing be removed only when being replaced. It is necessary to impact the bearing adaptor and bearings from the inside, which may leave impact marks. Because a new bottom bracket is being installed, this is usually not a concern.
The Trek® Bicycle Company has proprietary system is not a Shimano® Press fit, but does fall within the BB90 style of bottom brackets. Trek® Bikes offers the Shimano® Press Fit design on some models and their own proprietary system on the models. The Trek® system is reviewed at the end of this article.
In this threadless system, the cartridge bearing is housed inside a plastic (figure 1) or aluminum adapter/holder (figure 2). The outside of the adapter is nominally 41mm, slightly larger then the frame shell. The frame may be aluminum, carbon with an aluminum sleeve or just full carbon.
This bottom bracket system is serviced similar to a threadless headset. The adapters holding the cartridge bearings are a press fit into the bike. The bearing alignment references the shell face because the adapters press until the outer lip contacts the frame. It is assumed the shell faces are adequately parallel. Again, bearing adaptors are removed only when being replaced.
The bearing unit is removed using the BBT-90.3 Bottom Bracket Tool Set. Insert the bearing remover through the bottom bracket (figure 3).
Pull the remover through the bottom bracket (figure 4). Listen and feel for a click as the blades engage inside the bearing unit (figure 5).
Use a hammer to strike the tool end and drive out the bearing unit (figure 6). Reverse the remover tool and repeat by driving out remaining bearings. This will possibly leave cosmetic scarring inside the adaptor.
The bearings are a mild press fit into the frame. In aluminum frames only, it is recommend to either grease the pressed surfaces or to use a mild thread locker. This may seem contradictory, but the idea is to prevent future creaking if the cup were to move. The cartridge bearing is intended to be a permanent part of the adapter and the unit is replaced together.
To install the bearings, use a headset press such as the HHP-2 or HHP-3 Headset Press and a pair of the #669 bushings. The #669 bushings are included with the BBT-90.3 Bottom Bracket Tool Set. The bushing fit on the threaded shaft and do not fit over the hex section of the HHP-2 (figure 7).
Engage the bushing on each side, and then slowly press both bearing adapters fully into the frame (figure 9 and 10). Remove tool and bushing from bottom bracket. Install crankset (figure 11).
The Trek® Bicycle Company uses a proprietary bottom bracket design on some model bikes called the BB90. The crankset system is effectively the BB92 style, however the bearings themselves are different (figure 12). The bearings have no plastic retainer or cups. These bearings are a mild press fit into the bottom bracket shell. The cartridge bearing are 37mm outside diameter and 24mm inside diameter to accept the Shimano® Hollowtech® II cranksets. For SRAM® GXP® cranks, the left bearing has a 22mm diameter.
The cranks are removed with normal procedures. Use the BBT-90.3 driver to tap out each bearing. The bearing seat inside the shell cannot be removed.