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This is related to Worn out chainring or missing tension

Today the chainring arrived, but it lacks a pin. The pin is 15mm (0.59") long with a diameter of 4.1mm (0.16"). It is located in such a way, that it points away from the bike, but is covered exactly by the crank arm.

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I believe that its function is to prevent the chain to get stuck between the chainring and the crank arm, should the chain jump outwards from the chainring.

I fear that I bought the wrong replacement.

I bought this one Shimano FC-M523 / FC-MT500-3 10-speed Chainring for Chain Guards but maybe should have bought this one Shimano FC-MT500-3 10-speed Chainring (40 tooth variant). The latter was missing a proper picture (has only the 30 tooth variant) and was out of stock in all the shops I searched for.

Now I'm believing that the solution to my problem would be either to buy Shimano Chain Guard for FC-M4050 / FC-M4000 or just ignore the problem. Maybe you know of any other solution.

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I had the chain guard in the shopping basket, but when I read about its function on biking forums, they were saying that it's just to keep the trousers clean, so I ditched it. But maybe it does have the function of the pin, but all around the chainring, which sounds like a solution.

Any comments or recommendations on what I should do? I wanted to assemble it today so that I can use it tomorrow, but I guess I'd be OK with waiting a couple of days to have a proper solution.


Update: The wear of the chain drop pin is as follows, which leads me to believe that just ignoring it probably isn't a good solution.

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New idea: Would going to the hardware store and buying a screw with nut a reliable option?

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    Point of information: that's the chain drop pin. You're correct about its intended purpose. – Weiwen Ng Dec 1 '20 at 17:43
  • Lazy answer - can you move the pin from the old chainring to the new one ? Does the new one have a hole? – Criggie Dec 1 '20 at 21:49
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    @Criggie I tried this, but it doesn't seem to be easy and will most likely damage the pin. I used a nail and a hammer and gave it a few hits on the backside, but the pin has been spread under pressure by a machine, so that the spread part of it neatly expands the diameter of that section to hold the pin in place. I can rotate the pin, but to remove it, I would very likely need to break the deformation which holds the pin in; then I wouldn't have anything to keep it in place on the new ring (maybe epoxy glue would help?). I added a photo of the other side of the pin. – Daniel F Dec 2 '20 at 8:36
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You are correct, the chain guard’s purpose is to keep your trousers clean. I guess in this case it also prevents the chain from getting thrown over the chainring or it fills the gap between chainring and crankarm to prevent the chain from getting stuck there.

You only need the pin or chain guard in case the chain gets thrown over the chainring.

If you adjust the front derailleur properly this shouldn’t happen. I’d just use it the way it is. You could try gluing a piece of foam or wood into the narrow part of the gap (if it’s narrow enough to catch a chain) just to prevent the chain from getting stuck there.

Edit: Regarding your idea to use a screw+nut: The problem with a screw+nut will be that the nut and screw head will collide with the chain. It could even be that the wear marks in your picture were caused by the chain in normal operation if the “valleys” of the chainring were worn deep enough.

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  • Thanks. I added close-up photos of the wear of the pin. It looks like it has been useful a couple of times. – Daniel F Dec 1 '20 at 18:50
  • I was thinking the same about the head of the screw being in the way. I just made a comparison image new vs. used in another question. Actually, IIRC, I think that the chain got thrown over because it jumped out of the worn chainring when I rode the bike last Friday. At the end of this question is the comparison "photoshop": bicycles.stackexchange.com/questions/73629/… but I don't think that the valleys have become deeper (the scaling may be a bit off) – Daniel F Dec 1 '20 at 19:45
  • I will do it without putting anything there, as you say that this is most likely to be okay. Then, when I order another part, I will add the chain guard to the order and install it. Because just placing an order on it now would double the cost of the chain guard, which is not worth it. Thank you for your help. – Daniel F Dec 2 '20 at 10:12

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