To widen Hyro’s gear range, I opted to complete his Shimano 105 drivetrain by having the FC-5750 crank installed. While cleaning and degreasing the outgoing FSA Omega crank, I took a closer look at it.
For comparison purposes, I have another Shimano crank on hand, the non-series FC-R565 unit. While it doesn’t belong to any groupset, it’s almost identical to the 105 FC-5750 crank. The main difference is the R565 has solid forged arms, while the 105 unit has hollow arms for some weight savings. I’d say the R565’s chainrings are better looking due to the polished machining, but otherwise they’re identical to the 105 crank’s.
Between the FSA and Shimano cranks, the first real difference lies in the non-drive side crank arm.
All of Shimano’s Hollowtech II two-piece road bike cranksets are the “compression slotted” style. They have a non-drive side crank arm with a slot in the middle. When installing the crankset, into this slot goes a special hook-shaped “stop plate,” which is inserted until it seats into position.
Shimano also makes use of a large threaded cap, which threads into the crank spindle itself. This requires a special 8-point star tool to tighten properly. The purpose of this cap is to pre-load the bottom bracket bearings by compressing the two crank arms closer together, eliminating side-to-side play from the whole crankset.
After that, two opposing pinch bolts are tightened to 12-14 Nm to secure the non-drive side crank arm to the crank spindle. It’s a lot like how you would adjust a threadless modern headset: set bearing pre-load first, then tighten pinch bolts.
FSA uses what I think is a more elegant “self-extracting crank” style. Many of FSA’s two-piece cranksets, such as the Omega, make use of self-extracting crank bolts, which are semi-permanently inset under a retaining ring. To remove the non-drive side crank arm, you simply turn the crank bolt outward with a beefy 8 mm hex wrench, until it catches on the retaining ring and the crank arm eventually lifts itself straight off. This replicates the effect of a crank puller tool on an older square-taper crankset, but everything needed is part of the crank itself – no proprietary tools needed. Very neat.
The crank bolt does require a lot more torque in order to secure the crank. FSA quote 38-41 Nm. For reference, that’s the same amount of torque needed to secure a cassette lock ring onto a rear hub. Also, FSA makes use of two wavy spring washers that go in between the non-drive side crank arm and its bottom bracket bearing cup.
The second main difference is something I’ve mentioned before: the spindle – or specifically, its diameter.
All of Shimano’s Hollowtech II two-piece road bike cranksets have a steel spindle 24 mm in diameter. The Omega BB86 crank, very strangely, has a 19 mm spindle. You can see both cranksets’ spindles and the difference between them. It is this that makes the Omega BB86 crankset a bit of a dead-end product, because bottom brackets for a 19 mm spindle are almost impossible to find unless you have a factory-level hookup with FSA, it seems.
Or you could buy the sealed cartridge bearings, then press them into the cups…which risks damaging them.
Let’s take a closer look at the bottom bracket parts that it does come with.
The bearing cups have an external seal to further protect the sealed bearings that are inside them. I haven’t had problems or roughness with the bottom bracket so it seems these work quite well for bearing longevity.
After wiping off the dirt, I gave the inner races of the bottom bracket bearings a little spin with a finger to feel for any grittiness or roughness while moving. Even with serious mileage the bottom bracket is still fine.
After cleaning and learning all I can from the Omega crank, I strung along all the bottom bracket parts and the wavy washers along the spindle so that none of them will get lost.
All of the shoe rub marks on the crank arms aside, as of this writing, this is still a perfectly serviceable crankset, down to the bottom bracket bearings. While ultimate shifting isn’t as good as a Shimano crank, after almost two years of riding, I have a better appreciation of the close-coupled 46/36T chainrings as the 10-tooth gap helps with cadence control. While the 46T big ring restricted me to 48 km/h on flat sprints, I found it very usable for everyday riding.