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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?
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.
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.
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.
