SlvrDragon50

My left crank is constantly coming loose. It takes a few months, but I think something needs replacing at this point. I started reading into the problem more, and I did not realize that once the crank arm falls off once, it's basically toast? I have an FSA MegaExo + FSA Gossamer Crank. I've actually had the crank arm fall off twice. I only commute with this bike (<2 miles) so I really don't want to go drop $170 on a Shimano R8000. I was hoping to maybe just replace the FSA MegaExo since it'd be a much cheaper replacement, but if the fault lies with the crank then the problem will probably just reoccur a few months from now. I suppose I could just go for a 105 crank since it's a good bit cheaper with marginal weight differences. I was just wanting to get Ultegra everything since I already have Ultegra shifters + derailleurs.

trailangel

Find a new left crank arm and replace.

Eggman84

I know it doesn't directly answer your question (what to buy) but why not save all your $ and just tighten the crankarm on a regular basis, rather than waiting for it to fall off. Do you wait until your rims are bottoming out on the road before you put air in the tires?

base2 

In the late 90's I fixed a left crank arm once. I used a block of wood and a sledge hammer. I pounded it on GOOD, then grabbed a cheater bar for the fixing bolt that had red lock-tite on it. It never moved again. At least until I had the shop do a complete install of a road bike crank set on my mtn bike.

I think the shop mechanic still is cursing the day I walked through his door.

SlvrDragon50

Yes, that is what I do now, but I'm pretty sure a proper functioning crank + BB should not loosen in the first place which is what I'm trying to fix. I did come across a post where FSA was trying to do some new Loctite 641 compound on the BB spindle + splines that seemed to work, but I'm not sure if that is the correct solution if the cranks have already fallen out and have wear. 641 is a cheap solution, but I'd like to get it right the first time.

pdlamb

If the crank has loosened to the point it's fallen off, I'd expect it to be toast. Left crank is replaceable, so replace it. This time, make sure the interface is squeaky clean and crank the ever-lovin' .... out of the bolt when you torque it on.

jforman

Try this: use plumbers teflon tape around bolt and crank connection. I've used it on bottom bracket to keep cartridge in the frame

SlvrDragon50

Got it, will give that a shot.

Where do you get individual crank arms? I'm only finding crank arms used on eBay. Was reading that crank arms cost nearly as much as a full crankset so might as well buy a set.

jimc101

FSA Gossamer cranks (24mm spindle Mega Exo BB ), have 3 bolts on the NDS (LH) crank, the pre-load, and the 2 pinch; pre-load first, then do the pinch bolts up alternating till tight, suspect that you are having an issue with the installation rather than the crank is damaged (or was to start with) I have had issues with FSA Gossamer crank NDS loosening before, as the installation is a little different to Shimano, but once you know what your doing, it's an easy process.

Several of the post above offer suggestions applicable to square taper cranks, not external BB cranks, teflon tape will make no difference when fitting an external BB crank, and you probably won't be able to get the 5mm bolts into the crankarm on with any tape on them (not that they should ever need to be fully removed from the crank arm, just loosened, possibly the pre-load, but this doen't need to be done up overly tight in the first place; it can be fitted dirty (if you grease the spindle when fitting, by the time you get to installing the crank arm, it will normally have got some dirt on it), and this won't make any difference, again, this would be an issue for a square taper setup; and don't even think about 'crank the ever-lovin' .... out of the bolt when you torque it on.' as the pinch bolts (yes 2 of them, it's an external BB, not square taper) have a torque spec of 10.8-14.7Nm / 95-130in-lb which isn't a lot. if you torque them down really hard as suggested, they will probably snap, again, this would be more applicable for square taper, with a single larger bolt, when requires more (but still a specific) torque.

No mention of the age of the bike, you mention R8000, if the bike has this fitted for the rest of the components, the crank/bike should still be under warranty, as R8000 has only been out for just under a year now, which would meant that any bike fitted with it should be less than a year old.

gsa103

^^^ This ^^^
Gossamer is a spline type interface, as long as the splines are still good there's no need to replace.

With older square-taper cranks, when it fell off, it typically rounded the edges of the square, meaning it was impossible to ever stay tight. There's a reason manufacturers moved to spline interfaces.

SlvrDragon50

Okay, I definitely didn't tighten 3 bolts so something is wrong with the installation. I was a little surprised to hear splines would get damaged simply from the crank arm falling off once, but I'm really not familiar with bike mechanics. The bike is probably about 2-3 years old now running Ultegra 6800 components everywhere except for the BB and crankset. I'll pull the crank arm and take a look at the splines and then reinstall it all.

Thanks! The FSA crankset has honestly been okay other than this loosening issue so if this works, I'll keep it in.

Wait... I'm watching this video:

I don't see any other bolts besides the single hex bolt.

cpach

FSA used two different designs for different generation Gossamers. One was a pinch bolt arrangement like Shimano, the other uses a self extracting bolt like yours. https://www.stilen.com/bike_stuff/Meg...l_040820-1.pdf is the appropriate service manual. Note that the torque spec on the crank bolt is pretty high--350~400 kgf-cm (35-40NM). I'd try seriously leaning on the crank bolt if you haven't done so already. If you have proof of purchase on the bike you may be able to have a shop pursue a warranty as apparently they have a 10 year warranty on crank arms.

zacster

FSA cranks from that era were notoriously prone to loosening bolts. I have an Megaexo SLK and it does the same thing. My MegaExo BB is going bad, so I'm looking to replace that, but also thinking of replacing the whole thing, crank and BB. MegaExo BBs that are specific to this crank are not easy to find either. Mine is BSA threaded and surprisingly the Italian ones are available but the BSA isn't. I tried ordering a BSA that was listed but when I placed the order it then showed as unavailable.

My bike is all Campy. The original Chorus crank was square taper, and that seems so old school. I put that crank on an even older bike that had square taper and it has been great there for years. I look at Ultratorque but there isn't a compact available like that. I may have to put Shimano on my Italian bike, ugh.

zacster

And with my comments above I decided to ditch the FSA after 10+ years of frustration with it. I'm going with a matching Chorus 10 Ultra torque to the rest of my groupset. I think it was the only component that changed in the group. I found a 50/34 in a 170 length. I like that they are compatible with BBs of all types, BSA in my case. FSA just had too many incompatible variations.

Kovkov

zacster

That's why my old school Chorus crank is now running on my 40 year old steel bike! I was able to put it on a Zeus square taper BB without any further modification.