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My crank arm fell off, and I lost the fixing bolt. I had the crank arm reattached, and my friend tensioned the arm using the bolt temporarily removed from another bike, and since returned.

Do I need to buy a crank arm fixing bolt? And if so, which bolt do I use for a 2012 Specialized Sirrus Pro?

bolt side top

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  • Do you have to return the bolt to your friend or what is your problem with your current situation?
    – Vladimir F
    May 16 at 18:59
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    If the bolt is missing the crank arm will come loose again. Check with a bike shop -- a good one will have a suitable bold in a junk bin. May 16 at 19:00
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    So is the bolt now there or is it not?
    – Vladimir F
    May 16 at 19:02
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    @VladimirF Yes, he took the bolt back.
    – Neil G
    May 16 at 19:06
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    @NeilG Take a picture or tell us the words on them. The confusing thing here is that the bolt in your picture is the preload bolt for FSA Gamma/Omega Mega Exo cranks with adjustable preload that are then secured on to the spindle with pinch bolts. That's why the torque numbers on it are so low. But, that bolt is larger than the Powerspline BB your bike is listed as coming with, which takes a normal M8 crank bolt. That kind of crank very much needs its crank bolts. May 17 at 4:55
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Your crank is secured by the two pinch bolts. The bolt on the side is a preload adjustment bolt.

In a corner case scenario, if somehow the pinch bolts failed or were ridden loose or the spline interface cracked, it's possible to imagine the preload bolt keeping the cranks on such that you could bring the bike to a safe stop. So saying it's completely unimportant to safety may not be entirely accurate. But most of the time, with the crank in good condition and the pinch bolts torqued, it's safe to ride without the preload bolt in place.

You need it to keep the cranks adjusted and they are available by themselves, so it's probably worth procuring one.

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  • I figured out that I have a "FSA Omega compact" crankset. But I can't figure out if I need a QR-17 or QR-18 fixing bolt. The non-compact one needs the QR-18. The QR-17 is for "no pinch bolts". Do I have a no pinch bolt?
    – Neil G
    May 18 at 1:00
  • I believe the one in your pic you said worked is ML-131. If it fit everywhere when you did that, it's probably the one you need. FSA does have exploded diagrams if you look around, googling by the model code stamped on the cranks. May 18 at 3:07
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If the bolt in the photo is the same as the one that’s missing, it’s to preload the bearings before tightening the pinch bolts on a HT2-style crank.

If that’s the case, I’m not sure what consequences, if any, are likely from riding with just the pinch bolts and no preload bolt. I would think it’s similar to riding without a headset cap in that once the pinch bolts are set, it’s not completely necessary, but I’m not 100% sure it’s safe for the parts on a crank- there might be some repercussions I’m not thinking of.

I wouldn’t deliberately ride a HT2 crank sans preload bolt any longer than it takes to get to the shop for a replacement, but the implications of riding without it are not the same as for other styles of cranks where the fixing bolt torque actually holds or presses the crank onto the spindle.

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  • Thanks! That's exactly the same comparison my friend made: he said it was like a headset, but I didn't really understand.
    – Neil G
    May 17 at 0:40
  • I'll try some bike stores, but just in case, can you identify what words I should search for on Amazon?
    – Neil G
    May 17 at 0:41
  • Pisco is right on this. The original cartridge BB has been replaced with an "external bearing" BB. On the latter, just as with a threadless headset, the end cap bolt is used to first slightly compress the whole stack; the real work of clamping is then done by pinch bolts which secure the crankarm in place, locking in the slight compression of the stack.
    – Armand
    May 17 at 0:44
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    @Neil G "fsa crankset bolt" on google seems to generate appropriate hits
    – Armand
    May 17 at 0:48
  • @Armand Like this one? If so, do I need M18, M19, or M20?
    – Neil G
    May 17 at 0:50
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Without any photos it's hard for us to figure out what exact bolt you need. The bolt screws into the bottom bracket (which likely has a slightly tapered spindle, hence the need for the bolt fastened to the correct torque).

Based on your Bikepedia link, it seems that your bike originally had an SRAM S150 crankset, which seems in the 2011 era to use a Truvativ Powerspline bottom bracket. The Powerspline in turn seems to use these fixing bolts: Truvativ Crank Arm Bolts M8 Capless for Square Taper and PowerSpline, 11.6115.115.000-2 Pieces

Caveat: Your bike's bottom bracket may have been changed to something else, even at the time of original purchase, in which case these bolts may not be correct. Don't forget to put a dab of grease on the threads before attaching the bolt, and use a torque wrench to get the correct torque.

Important edit: OP's photo shows his BB is NOT a powerspline cartridge, but rather an external-bearing BB, so the above bolt suggestion doesn't match his bike.

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  • Thank you for the excellent answer. I added a photo of a bolt that worked. Does it help?
    – Neil G
    May 16 at 22:48
  • @NeilG: The indicated torque (.7-1.5Nm) is far too low to hold the crank-arm in place while riding your bike. It may work on a static bike. There should at least be one or two pinch bolts to clamp the crank on the spindle/axle.
    – Carel
    May 17 at 19:51
  • @carel There are two pinch bolts holding it in place now.
    – Neil G
    May 17 at 20:06
  • (FYI, I updated the bike model; I was off by a year, sorry)
    – Neil G
    May 18 at 0:22

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