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I recently bought a Specialized Sirrus 3.0 and it came with a Shimano MT-210-2 crankset and a rear Shimano Sora 9 speed 11-34T. The left crank arm fell off twice, stranding me while riding, so I asked the shop to change out the crankset for something better and they replaced it with a Shimano RS510 which is intended for use with a 2x11 speed cassette. I rode it once and I thought it seemed pretty smooth, seems more expensive than the Sora at first blush, but should I request to instead have the crankset replaced with a Sora crankset since that would then be appropriately matched with the Sora 9 speed rear? Thanks in advance for any advice from a novice biker trying to learn all I can about the parts of my bike.

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The new crankset is fine. The only difference between an "11-speed" crankset and a "9-speed" crankset is a slight difference in chainring width; you can use a narrower chainring with a wider chain.

However, it is very worrying that you had a crankarm fall off—twice. That isn't normal for a correctly set up bike. If you bought this bike new from the same shop that sold you the new cranks, they should have been falling all over themselves to make this right at no charge. And also, if so, it does not give me confidence that they installed the new cranks correctly.

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    The shop doesn't install the cranks on a new bike, they oniy (should) check bolt torque. Bad qc at specialized or inferior components perhaps?
    – JoeK
    May 15 at 14:27
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    Yep - HT2 cranks falling off is a clear sign of improper installation. To be fair, after they fall off once, the splines are often severely damaged.
    – MaplePanda
    May 15 at 18:51
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Completely agree with what you are saying. To say I was upset and frustrated would be an understatement. But while the work from the shop has been shoddy, I always ask for compensation and they haven't objected, giving me the helmet and lock for free (~$170). They upgraded the crank for free and they will be swapping the whole bike out when they get a new shipment of them and I'll have them install the upgraded crank on that one when they give it to me, then get it checked out elsewhere just to be sure. The bike had a recall on the original crank but the above mentioned Shimano crank that came with mine was visually different than the original one on the specialized listing/recall. However, the recall was for the same exact issue with the left crank arm coming loose. Guess I'll have to wait and see how the RS510 fairs. It is definitely a much higher quality crank than the one I had but the left arm still attaches to the crank in the same fashion, bolted around the end rather than slip fitted over which makes me nervous. Thanks for responding, feel good about keeping it for now rather than having them swap it again. I Was just worried that they gave me something inappropriate/improper for my chain/rear setup.

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    I actually like the way Shimano clamps the left crank arms to the spindle very much. I’m surprised it’s possible to have a quality control issue where it comes off the spindle, you’d think that a sufficiently strong clamp can’t come loose.
    – Michael
    May 15 at 17:20
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    The Shimano HT2 connection to the axle is bombproof if you tighten it to the specified torques. If you have it loose and destroy the interfacing surface, it won't stay there any more whatever you try. BTW, the instructions on my bike said: "After the first 20 km tighten the crankset and also tighten the pedals in the crank arms. Check if crank bolts are properly tightened. Check whether left crank arm screws are tightened firmly when bottom bracket axle is integrated with right crank arm."
    – Vladimir F
    May 15 at 19:36
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    After the first few miles and rides, I always recheck the torque on the left crankarm's pinch bolts (takes a 5mm hex head). The torque on both these bolts should be between 12 and 14 Newton-meters. Use a torque wrench set within this range to make sure the torque is correct. Subsequent checks as part of a drivetrain clean and lube is wise, though I've not had any problems after the bolts have been--or returned to-- the proper torque after being ridden a short time. A key to the installation is bringing the bolts up to torque together, switching back to the other bolt after about 1 full turn
    – Jeff
    May 16 at 10:47

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