Ive stripped the thread on my Ultegra cassette lockring (11sp, 11-28t)and need one urgent. Ive swapped the one from XT mtb (10sp, 11-36t) cassette over to it, as a short term measure until I can get to the LBS, and it seems to work perfectly. Does this seem feasible, or will I die the moment I turn a pedal?

I would imagine they are identical

Pretty sure they’re all the same, I mean they all have to fit into a standard freehub, right?

You do get some variation between the different lock rings and from memory some don’t fit the bigger (13 + teeth) top gears.

As you’ve done a 11t – 11t swap I’d be VERY surprised if you have an issue.

They are all the same fitment, but the more expensive ones are lighter.

Same thread, etc., but different diams. I found out when i tried to use one from a 12T cassette on an 11T, and the lockrig fouled the chain so it wouldn’t sit down on the teeth properly.

Aye, I had that too. But 11T lockring on 12T cassette seems fine.

Half the weight difference between SLX and XT used to be the lockring!

The main difference between an XT and SLX cassette weight is down to the lockring.

Put a 11T XT lock ring on my 11T SLX cassette with no issues. Someone will be along now to say exactly how much weight that saves 😀

About 10grms difference between alloy and steel lock rings 😉 and as above some compatibility issues between 11 and 12T.

Myself, I’d very happy to have steel lockring, because, as you’ve noticed, alloy ones strip.