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I was cleaning up my vintage bike crank arms, and somehow ended up losing the nuts/bolts that would screw my crank arms back into the bottom bracket.

Started asking my bike shops only to find out this kind of bottom bracket is quite old since you need a nut that screws on the outside instead of inside as are most vintage squared taper bottom brackets. Here's a pic of what I have:

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I found online also the picture of the nuts that I lost:

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My problem is that I can't find these at any of my local shops.

Question: can I replace these with just regular nuts + washer I can find at hardware stores?

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EDIT note how the nuts had a rough surface in the washer part making me think that just a regular washer wouldn't do:

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  • You could have 8 spares for when you lose the next pair amazon.it/…
    – Warren Burton
    Apr 14, 2022 at 8:31
  • Aside - your second photo says "M10 x 125" when it really means "M10 x 1.25" the second number is the "gap between adjacent threads"
    – Criggie ♦
    Dec 17, 2022 at 20:13
  • Do note that it's not unusual for one of the nuts to have left-hand threads.
    – Daniel R Hicks
    Dec 17, 2022 at 23:34

5 Answers 5

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I don't think a regular nut and washer would be a good choice. The washer would likely deform pushing against the crank arm. And the rough surface you mention is there to help prevent the nut from loosening on its own. You may be able to find a "flange nut" at your hardware store, which I believe is the generic term for this type of nut. On a side note, it's really sad that your local shops don't have basic repair parts like this in stock. There are still a ton of bikes on the road with this type of bottom bracket.

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    These aren't a "basic repair part", because they will outlast the bb they are sold with. You have to be a bit dull to lose them before the bb has finished its useful life, so it's a bit unfair to take aim at the local shops
    – Swifty
    Apr 13, 2022 at 22:48
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    I disagree. I used to work at a bike shop and we replaced these for customers fairly regularly.
    – Noah Sutherland
    Apr 14, 2022 at 23:03
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It's a flanged nut. These are absolutely normal fasteners, but maybe not quite common enough for the average big-box store.

Try a local specialty fastener outlet instead, or look into a reseller with more depth, like Mcmaster Carr or similar.

Your other option is to have a cleanup in your working area and locate the missing nuts.

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If you can’t locate the right nuts you can use the inner bolt version of a square taper bottom bracket. The M8 bolts for that are easy to find. Your existing cranks will still mount fine assuming you get the right standard which looks to be JIS from the photo.

Assuming your BB shell is 68mm a Shimano BB-UN300 will drop right in. You’d need the spline tool also. I’m not sure if your existing crank puller would be compatible.

I agree with Noah, use the correct nuts. You don’t want the nuts coming loose and destroying your cranks.

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Go to your local used bike and parts shop/store. The bike co-op near me has a coffee can full of these that they'd probably sell for $.25 a piece.

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  • Although the original questioner stated their local bike shop did not have any, there are other bike shops, so persistence can yield results.
    – Ted Hohl
    Dec 17, 2022 at 20:40
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You cannot get them from hardware store. Yes one side is a normal flanged nut, but the other side is a reverse thread. These are extremely rare. are you sure about thread pitch ?

A better idea is to get a complete axle, they come with nuts. You also have to measure axle length, as they come in different sizes. Yours is probably 6T.

These are still used on very low end bikes, and axles can be found on ebay, or Amazon. They are very cheap probably around 2 dollars.

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