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I recently posted a question about changing my chainset, I was advised on this SRAM chainset which I could use which would fit my square tapered bottom bracket.

I have attempted to fit the new chainset and managed to get the crank with the chainrings on onto the bottom bracket spindle. However, I am struggling to get the other crank onto the bike because I cannot seem to screw the other bolt that came with the chainset into the bottom bracket with or without the crank. It screws around but does not screw in. This is preventing me from getting the other crank onto the bike. I am not sure if I am doing something wrong, could buying grease for the bolt help attach it or should I contact the seller and try asking for a new bolt?

Here are some photos of the bottom bracket, the problem bolt, and the crank which I added to the bike:

enter image description here enter image description here enter image description here Crank which I successfully added

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  • That's clearly the wrong size bolt.
    – Daniel R Hicks
    Commented May 29, 2019 at 23:05
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    @daniel although the matching, right hand bolt went in ok?
    – Swifty
    Commented May 30, 2019 at 7:53
  • Try threadding each bolt into the other side.
    – Criggie ♦
    Commented May 30, 2019 at 9:45
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    One thing you should try is use a piece of wire or a thin screwdriver to dig into the hole and see if there is any gunk buried at the bottom of it, preventing the screw from going in further. Probably not, based on the picture, but it can't hurt to try.
    – Daniel R Hicks
    Commented May 30, 2019 at 11:41

2 Answers 2

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I suspect your bottom bracket has a damaged internal thread, and neither bolt will thread into the bad side.

Two fixes - you could try using the right sized tap to chase and clean the threads, but this depends on there being sufficient metal remaining. Plus taps are expensive.

Second option is to replace the whole bottom bracket cartridge with a new one. That's got a cost, but the existing cartridge certainly has some mileage on it.

I'd fit a new BB if I didn't own the right tap already. They're surprisingly cheap (generally)

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  • BB spindles being hardened, tapping is probably impossible.
    – Carel
    Commented May 30, 2019 at 12:25
  • @Carel true - its more cleanup and chasing the existing threads. If the two bolts have a problem on one side but not the other, then its definitely the side not the bolts at fault.
    – Criggie ♦
    Commented May 30, 2019 at 20:20
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The picture of the left side of your bottom bracket without the bolt is not real clear, but I wonder if there's a round obstruction partially blocking the opening and preventing the threads of bolt and BB from engaging. Normally there is a mm or 2 of unthreaded counter-sink at the beginning of the spindle's hole prior to the threads beginning. While it could be a photographic anomoly, I could see some of that counter-sink--or some threads themselves--galling and upon removal of the original bolt, get spun up at the opening, creating a partial blockage. That would cause the symptom you describe.

At any rate, here's example of a replacement bottom bracket (not a recommendation). You'll need to select the correct length of spindle, but as you see, they're relatively inexpensive and replacement is appropriate regardless of the reason the bolt won't engage. here's an internet image of the end of a BB-UN26

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