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I have an old road bike I want to get a single speed crank for but don't know what fitting the bottom bracket has.

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    That's what's known as a "normal, everyday crank" .. in 1960.
    – Daniel R Hicks
    Commented Dec 11, 2018 at 13:14

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The bottom bracket spindle is for cottered crank.

The bottom bracket looks like threaded one, so it is likely that it can be replaced with square taper or external cup bottom bracket. To tell which thread the bottom bracket has, we'd need to know the exact dimensions.

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    Also, if the bottom bracket standard turns out to be obsolete, it might still be possible to only replace the axle with a square taper one.
    – Wsal
    Commented Dec 11, 2018 at 16:29
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    Here's a crib sheet for threaded bottom-bracket standards. In order of likelihood, it's probably British, then Italian, then French (unless it's a French bike).
    – Adam Rice
    Commented Dec 11, 2018 at 18:07
  • Thanks for the advice! I've ordered a couple of suitable tools for removing it and will see if I can replace with a sealed cartridge or just replace the axel. Getting a knackered old bike as a project is a great way to learn about bike maintence etc, although a £25 bike quickly turns into a lot more with tools and bikes it's still a great investment in terms of learning and keeping myself entertained.
    – Andy Stannard
    Commented Dec 13, 2018 at 10:38
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As ojs says, you have a spindle for cottered cranks. Cottered cranks basically do not exist anymore, unless you look for 'vintage' parts. To fit a new single speed crank you need to replace the bottom bracket bearings and axle along with the crank.

Your bottom bracket shell is am internally threaded kind. The most commonly encountered standard for thread width and pitch is known as 'English' or 'BSA'. On an older bike this may not be the case.

If your bike is English threaded there are lots of options available. Probably the best option is a threaded internal cartridge BB bearing with square taper or splined crank interface. Square taper single speed cranks are plentiful and inexpensive. This also allows you to choose the spindle length to get the chainline you need.

If you can get the old bearings out and ca get hold of an old English threaded BB you could try it for size. I say this a bit hesitantly though because I have a feeling there are some threadings that are close to but not exactly the English standard.

Update: Sheldon Browns page on threaded BB shells for reference: https://www.sheldonbrown.com/cribsheet-bottombrackets.html

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