Bicycle Mechanics - I sheared off the crank arm bolt

I have an older FSA SLK crankset, and the bolt has always been a problem. No matter how hard I try it just wouldn't stay tight. Well today on my way home from a fifty mile ride it had come loose for the second time today, and I had tightened it before I left too. This time though when I tightened it it suddenly started turning free, but what had actually happened was that the flange sheared off, making the bolt useless. I took the long silver sag wagon the rest of the way home.

My question is whether I did any damage beyond the bolt? I already ordered a new bolt and extractor set from FSA but will I still have problems?

I'd say no to the other damage. Essentially what has probably been happening is the flange has been cracked or yielded for some time and that could explain why it was coming loose continually. Also you could try blue loctite.

What torque are you tightening the bolt to? I believe the spec is 40nm +-. I suppose Loctite might help, but the assembly instructions don't call for its use.

Is this an older 'square taper' crank? check the taper hole on the arm it maybe rounding out, cranks that repeatedlt come loose is sure sign cause of the hole rounding out.

Unfortunately I don't carry a torque wrench with me on the road, but the bolt does have the recommended torque on it, 350-500 kgf cm (which I guess is 35-50nm, so yes 40+-). I do have a torque wrench but I never used it, in fact I think I bought it when first assembling this bike. I'll learn how to use it once the new bolt comes. With a hurricane bearing down on us, and then my travel to Memphis, I won't need the bike this week anyway, and besides, that's what the 3 other bikes are for.:)

The manual also says to not flatten the wave washer, but I don't know how I would know I'm doing that.

Is this an older 'square taper' crank? check the taper hole on the arm it maybe rounding out, cranks that repeatedlt come loose is sure sign cause of the hole rounding out.

No, not a square taper. This is a mega-exo 2 piece crank, a more modern design. I vaguely recall the spindle being splined to take the left arm, and permanently attached to the right arm.

And the rounding makes the crank come loose, which causes the rounding, and so on...

Unfortunately the single bolt design is very poor design. To work properly the wavy washer needs to be semi- compressed to hold the bolt tight. If you have overtightened the bolt then the washer probably has been over compressed and can't do it's job any more. Further, the design requires a pretty tight fit on the splines. If it was come loose and rattled around at all the arm is probably toast.

Edit: you might want to see if you can purchase an updated FSA left crank arm. I don't know if it will fit (it should) but it has a clamp style that pretty much guarantees it will not come loose.

280822

It's chinese, boss.

It's done.

Here we go

No, not a square taper. This is a mega-exo 2 piece crank, a more modern design. I vaguely recall the spindle being splined to take the left arm, and permanently attached to the right arm.

And the rounding makes the crank come loose, which causes the rounding, and so on...


Oh OK that is way to new for me.

Unfortunately the single bolt design is very poor design. To work properly the wavy washer needs to be semi- compressed to hold the bolt tight. If you have overtightened the bolt then the washer probably has been over compressed and can't do it's job any more.

Incorrect. The wavy-washer only takes up axial play after the crank bolt is tightened.

I have an older FSA SLK crankset, and the bolt has always been a problem. No matter how hard I try it just wouldn't stay tight. Well today on my way home from a fifty mile ride it had come loose for the second time today, and I had tightened it before I left too. This time though when I tightened it it suddenly started turning free, but what had actually happened was that the flange sheared off, making the bolt useless. I took the long silver sag wagon the rest of the way home. My question is whether I did any damage beyond the bolt? I already ordered a new bolt and extractor set from FSA but will I still have problems?

Classic case of putting off getting to the bottom of a known problem until it bites one on the ass, right near the wallet! If the item is a square taper crank.. I have seen a few where they had been over-tightened causing the hole to enlarge or round out. This leads to the spindle no longer fitting perfectly and that leads to the arm wobbling a bit with each stroke and thus the bolt works free. I guess similar could occur on other types. In the cases I had run into, the only fix was a new set of crank arms and I always also put in a new spindle and bearings. I strongly recommended the owners stop at their local KMart and get a $15 torgue wrench to protect their new $150 investment. Some locite is also a good idea...