FSA crank + BB combination too tight

Question

I have a bike here fitted by OEM with:

• FSA Omega Adventure MegaExo crank
• FSA BB-4000

No maintenance or mods done around BB area on this bike. If you are curious it is Pinnacle Neon 5 2019-version. Only around 5000 km on it from new.

The crank never really spinned well from the start on that bike. Without the chain in place it does at most 1/4 of a turn before coming to a stop. I have been asked to look into it recently. I initially thought to write it off as FSA lower-end cranks being garbage in general... But eventually took it apart to take a look. The findings are interesting...

The NDS crank arm has a self extracting mechanism (like most FSA Crank arms do) and against the expectations it worked pretty well. I was able to pull of the NDS crank arm and then take the DS arm out with the axle (no mallet needed). I checked the BB in the shell without axle and it appears to be silky smooth. I did the usual TLC on the BB and crankset:

• Removed BB from the shell
• Cleaned the threads on BB, shell, self extractor nut, spindle and retaining bolt.
• Greased the threads
• Greazed the axle
• Put everything back together

The crank spins silky smooth until the point when I tighten the NDS crank arm retaining bolt. FSA spec says 40-50 Nm. The crank stops spinning properly around 10Nm which is way too low to be secure in my view.

There is a wave washer between NDS crank arm and the BB face which is supposed to preload the bearings. This wave washer is fully compressed and looks flat at 10Nm. Anything above that and I am compressing the bearings. By the time I reach ~40Nm the cranks is back to the same state as pre-maintenance - it does about 1/4 turn without chain in place.

From the situation described above I am very confiden that the issue is simply with the NDS crank arm sitting too close to to the axle centre when tightened to correct torque setting. Bearings are overloaded and cannot spin free. Could be a manufacturing tolerance issue. There is definitely no alignment problem as the crank arms spin well when the retaining bolt is at 10Nm...

I am really surprised as to how bearings did not get damaged after riding ~5000km in this state but this is off topic.

Any ideas on how to cure this problem? Anyone has experience with FSA cranks?

If there were any spacers, I would start by removing them to allow more room for that crank arm and wave washer, but there are none... Either none came with the crank, or not fitted by whoever was assembling it. And rightly so - spacers would have made it tighter.

To aid the discussion here is an illustration from FSA manual (not the same but very similar crankset:

Ironically in the same document I find this (which clearly does not help):

UPDATE 2021-05-11

Measured the frame with vernier caliper. It's 67.93mm so pretty close to expected 68 (allowing for the tool and measurement accuracy).

Answer 1

Ok, so I got this sorted!

Thanks to everyone for input. Posting the solution here in case someone else is facing the same problem.

The answer is inspired by @Nathan Knutson 's post.

@Nathan mentioned a transparent scuff washer... And I did not remember seeing one when taking the crankset off. So I took it all apart again and specifically looked for it. There is indeed a transparent plastic (or nylon?) washer that was tightly pressed into a small groove on the inner side of the NDS crank arm. It was flush with the surface and did not come off when I was cleaning the crank so I assumed it is part of the crank arm and not removable.

Now I removed that washer and it freed about 0.3mm for the wave washer to expand. Asessembled, torqued to recommended setting, crank spinning well and I can clearly see that the wave washer is no longer flat - it now serves its purpose and preloads the bearing as intended.

There is a slight concern that removing the scuff washer will make the wave washer rub against the crank arm and scuff it over time but for me this is a minor concern as this crankset is cheap and I was close to replacing it with Shimano or SRAM dub or anything else with adjustable preload... Now it will serve a little bit longer before being discarded.

For all those who assumed misalignment of the BB shell - this is a valid assumption but wasn't my case.

Answer 2

The next thing to look at after the things you've tried is the BB shell. The shell face-to-face dimension being out of spec could cause this, as could any distortion from welding. The two problems are usually related since BB shells start out as machined parts; the idea that some are just made too long or short is possible but mostly unrealistic. Getting it chased with piloted taps and faced at least eliminates those possibilities. You mention not wanting to mess with this in the comments, but here's the catch: it's true that Shimano cranks aren't sensitive to pure width tolerance problems in the same way due to their manually set preload, but if the problem you're having now is from distortion/alignment, that can affect other cranks in the same way. Meanwhile, you're confident taking it all apart. Bringing a bare aluminum threaded shell to a shop and asking for this service is about the fastest possible interation of it. So there's a lot to be said for just doing it and going from there.

If you did find after machining that spacers were necessary, there are translucent FSA scuff washers for the 19mm cranks, but they can be a little hard to chase down. If it were a facing or distortion problem, it is possible you may find yourself needing some all of a sudden, i.e. if by the time it's square the width gets knocked down some.

It is true that spending much to keep these particular cranks going is dicey. However, you may also decide that's a problem for a year when there are more cranks to buy.