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The crank arm on my Decathlon Triban 520 became loose during a cycle. On inspection the crank arm bolt is missing. What bolt do I need as a replacement? (Or how can I figure it out?)

The manufacturer indicates the groupset is Sora R3000, I don't know if this includes the bottom bracket though. The external/internal diameters of that threaded portion are 24mm/19mm

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On the Sora R3000 (and many modern cranksets) the axle, right hand crank arm and chainring spider are all one piece. The left hand crank arm fits on the splines on the axle, and is held in place by two pinch bolts. The thread you see in the hollow axle is for a plastic cap that is finger tightened to provide slight pre-load of the bottom bracket bearings.

See this Park Tool webpage, section 3 'Two Piece Compression Slotted Cranks' Also, here's the R300 crank installation manual.

If the crank was loose, the pinch bolts must have loosened. It sounds like you lost the pre-load cap, but check that the pinch bolts are still there also.

Now the bad news. When aluminum cranks come loose on a steel axle, the large pedaling force can very quickly deform or wear the splines in the crank arm. Your left crank arm may be ruined. This seems likely as there are aluminum filings visible in your picture. You can try re-installing the crank arm, but it it likely to come loose again. There's a link to a video on the Park Tool page that shows you how to do that. Of source you can also take this to a reputable bike repair show and have them assess the state of the crank arm.

The bottom bracket is separate from the right hand crank arm and axle. It may seem one piece, but the axle is quite a tight fit in the bearing races. Normally the end of the axle needs to be hit with mallet to get it out.

If you end up having to replace the crankset, the specific model of crank your bike has is the FC-R3030 (three speed, without chain guards). You may be able to find a used replacement left arm, but I'd definitely have a bike repair shop check the axle if you go that route.

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    Ah, that is unfortunate. When I noticed the problem I stopped to tighten up the pinch bolts but the crank immediately became loose again. I thought this was because I was missing the bolt, but if there is no bolt, it sounds like one or both sides of that mating is ruined. Thanks for the explanation. – frodoskywalker Apr 2 '19 at 0:28
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    @frodoskywalker The pinch bolts have to be tightened to the correct torque, which is difficult to do as a roadside repair. – Argenti Apparatus Apr 2 '19 at 0:36
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There is considerable amount of aluminum fillings and debris. The photo is not clear enough to determine if the crank axle is the primary source or if it is from the crank arm. If the crank axle shows any sign of wear it will need to be replaced. Unfortunately the crank axle is part of the crank spider assembly. While the right arm and the left arm/spider assembly are available as individual components they will likely exceed the cost of an entire crank assembly. This would include the ring gears, left side crank arm (this includes the axle), right crank arm and possibly the bottom bracket bearing assembly.

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