OK, just bought a Specialized Camber Expert 29-er fitted with a Roval Control Trail 29 142+ rear hub.

I’ve decided to make it 1×10 with a Hope T-Rex expander.
The T-Rex fits onto the freehub body fine, along with the rest of the cassette (SRAM PG1050). The problem comes when I put the wheel onto the bike…
When I tighten the axle it seems to press the T-Rex into the main hub body, which then means that the cassette won’t spin on the freehub.

Anyone seen this issue before? And any idea what I can do about it?

You are removing the 17t cog aren’t you?

I’d go with the option above but would also be sceptical of a special specialzed (non)standard.

Yep, 17T cog and spacer removed to add the T-Rex.
The cassette on it’s own fits the freehub, and the lockring tightens up. When the wheel is out of the frame like this, the freehub spins freely. When I put the wheel into the frame and tighten up the axle, it appears to push the whole cassette into the hub body, stopping it free-wheeling.

Try a 1mm shim behind the T-Rex?

you have got the correct Trex for the sram cassette and not a shimano one

Sounds like you’re missing a washer/spacer that sits between the freehub & inner hub bearing if the freehub & hub body are binding when you give them a little squeeze together.

Thanks guys. It’s definitely the SRAM T-Rex.
Sounds like a shim / spacer behind the T-rex is what’s needed.

A wee bit of Googling has found this, if anyone’s interested for future reference:
Wiggle sprocket spacer

Thanks all!!

OK then, an update to this…

I bought and installed the shim as per the above Wiggle link.

The cassette now spins freely on the freehub, but… the 11T cog binds agains the bike frame now. 👿

Any ideas?

A long shot, you did put the T-Rex ring on the right way round. Saying that, I’m not 100% sure you could even put it on the wrong way round.

Alternatively have you got arms line Popeye and are seriously over tightening the lockring

It sounds like what hob nob described, have you had the freehub off at any point?

I’d be trying the sram cassette on its own again, if only to rule out any issues with the freehub itself.

Did you buy the Hope ‘Sram’ Trex or the Shimano version?

It’s definitely the SRAM version of the Trex.

The SRAM cassette still works on its own. I’ve had to put the bike back together like this, so I’ve got something usable.

I’ve got a 1mm spacer as HobNob suggested that sits between the hub body and the Trex. When this is installed, the cassette binds against the frame when the wheel is in the bike.
Without the 1mm spacer, the Trex binds against the hub body.

Until a couple of months ago, I had been single for 2 years… but my forearms aren’t exactly like Popeye 😉

I might just have to man up and make do without the bail out 40T gear then…

Kin weird that is.