Any difference between the two or can I use an mtb bottom bracket with a road chainset?

they’re designed for different sizes of bottom bracket shell, but otherwise no.
i.e. it just comes down to the size of the spacer inbetween the bearings.

I thought mtb bb’s had a longer threaded section than road bb’s, i.e. more metal threaded into the frame?

PROLINE85

Some MTB bb’s have extended threaded sections but most are the same road or MTB.

The only other thing to consider is the threads in the road BB shell some are italian some are english. To the best of my knowledge all MTB bb shells are english.

Makes sense Rusty Mac, would imagine Saint and the like have longer threads. Had forgotten about Italian standard 😉

The plastic center tubes are different between road and mtb, but the actual cups and bearings are the same iirc.

Just removed a knackered Tiagra road bb this evening, as one bearing locked after 300 miles. Fitted an Uberbike one tonight and swapped straight over. My Tiagra cranks have a spacer fitted on the axle between the back of the crank to the BB. Mates Spesh Tiagra bike doesnt. I took the spacer out and used the alloy spacer rings a la MTB setup and runs fantastically smooth. Not tested outside as yet.

If you take the time to measure them, road HT2 cups sit 1mm further out on each side, whether that’s a noticable difference is up to you to decide.

I use a road ht2 BB on my mountain bike (cos they are mostly identical and it was cheaper). It just requires an extra spacer iirc.

I bought an mtb ht2 bottom bracket for my cx bike by mistake, fitted if with no issues and no spacers.