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bottom view what is the name of this nut, I need to find a way to loosen it .

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It's a spanner nut. There's a special wrench to loosen and tighten it but putting a slotted screwdriver in the groove and tapping it with a hammer usually works. it's a double nut so loosen one first, then the other.

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    Thank you. , I thought it was a more complicated but .. not sure why I thought that.
    – Charm_quark
    Sep 28, 2019 at 21:26
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    You can a find spanner wrenches of all sizes in a bicycle shop in the tool area. A pair of vise grips may work also but you may to use something to hold the top one while you loosen the bottom on.
    – Alaska Man
    Sep 28, 2019 at 21:50
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    That's got to be the US term for it. In the UK all nuts are 'spanner nuts', because a spanner is what we call a wrench ;-)
    – Tetsujin
    Sep 29, 2019 at 13:25
  • @Tetsujin In the UK, spanners are called spanners, and adjustable ones are called adjustable spanners... Spanner may be also used to refer to someone but that is not polite... Wrench is more American... And given I spent my apprenticeship in the UK “on the tools” I know a spanner when I see one.
    – Solar Mike
    Sep 29, 2019 at 18:07
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    @SolarMike I do know one US supplier that calls a tube spanner a "spanner wrench". It always seems like whoever named it is a bit of a spanner spanner
    – Chris H
    Sep 30, 2019 at 9:59
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I find using the correct “c” spanner, in fact I would use two together to undo that as it means no hammering to possibly damage bearings etc.

This method of using two nuts is used to set a limit or stop position - if only one nut was used it could easily move. A "nyloc" nut may also move but the two nuts locked together do not move. Some nuts can have a set-screw inserted to prevent movement but that is less secure than the two nuts...

Called a c spanner because it looks like a c but has a tooth at the end to engage in the slot on the nut.

two examples:

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    Thank you, I have honestly never seen anything like that before, The nut is for a machine from china. So it just baffled me.
    – Charm_quark
    Sep 29, 2019 at 18:45
  • Also called a hook spanner
    – Chris H
    Sep 30, 2019 at 9:58
  • @Charm_quark Depending on the kind of machinery, they are (or used to be) somewhat common. Not just in China, so don't be surprised if you see them again.
    – Mast
    Sep 30, 2019 at 14:13
  • @mast : Is there any advantage over the standard nut?
    – Charm_quark
    Oct 2, 2019 at 5:37
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    @Charm_quark see edit, but it provides a stop position which cannot move unless left loose.
    – Solar Mike
    Oct 2, 2019 at 5:42

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