It sort of looks too neat to be a crack so I thought it might just be the join, but I can't find anything similar on the other wheel of the pair. 🙁

I had something similar on a wheel of mine, I got it looked at and then replaced it ..
mine was awfully neat also

What rim?

Is it directly opposite the valve hole? If so it's the join.

I'ts just where it was joined when manufactured.

^ what wwaswas says

manufacturing join

Yup you poss find 3 other – where its pinned and jointed 🙂

It's on my Kaffenback, Planet X road wheels I think.

I thought it must be a join, but I literally can't feel or see anything similar at all on the other wheel. Is it possible it's the join but it's got smacked out of alignment a little?

What about this one?

That's clearly fine 😉

rorschach – the tyre will hold it together…

Usually, as I am sure most people know ;), rims are usually made out of 4 bits pinned and jointed, 1 piece pinned and jointed or 1 and welded 🙂

rorschach- you need anti-talcum powder on your tubes so they stick to the rim, and never deflate your tube or the wheel will fall apart.

I managed to do that on the wc decsent at the Dyfi enduro-it not only held together but it stayed airtight (tubeless) for the next 40km!!!!All I could feel was bump…..bump….bump through the seat on the fireroad bits!!!