I have a Mavic Crossride wheel that I want to use tubeless.
I've taped it and that seems to be working.
With the Mavic tubeless valve, the thread on the sealing collar does not screw down enough to compress the rubber o-ring seal.

There was a collar that was fitted into the rim's valve hole. But this has a slit in it, which prevents it sealing. So this is removed.

Any tips?

I've seen on the web that using an old Innertube valve with about 50p coin size of tube around it may work.

I had this issue with single-walled fatbike rims. I eventually bought some Super P-Nuts

https://problemsolversbike.com/products ... t_tubeless

but my first solution (which did work) was just to fit a couple of soft rubber o-rings on the valve before fitting it. I got a pack of mixed sizes from Halfords but you'll see a massive selection on the likes of Amazon

Thanks for that.
I tried various o-rings but failed. Shame, because the tyre sealed perfectly. Had to put an innertube in.

You could perhaps drill a larger hole in a tap washer and fit that beneath the valve.