Mounting Tailfin on MTB – UDH axle size question

I’d like to mount my Tailfin kit on my MTB.

On my gravel bikes I have a UDH bolt with a 12 mm hex interface, so the Tailfin thru-axle passes through without issues.

On my MTB, instead, the UDH bolt has an 8 mm hex interface, and the Tailfin axle won’t fit.

Questions:

• Are all UDH bolts interchangeable, or do MTB and gravel versions follow different standards?

• Can I safely replace the 8 mm UDH bolt on my MTB with a 12 mm hex UDH bolt, or are there compatibility/spec differences (length, thread, shoulder, material, etc.) I should be aware of?

Last point: the spec says 25 Nm for the UDH bolt, which already seems quite high.

In practice, I’m struggling to loosen it: it feels tighter than 30 Nm, or possibly seized/oxidized.

• Is this normal?

• Should I expect problems removing and reinstalling it with a standard torque wrench?

Any real-world experience appreciated, especially from people running Tailfin on MTBs with UDH frames.

Thanks!