Stocking up my spares box, I think I’ve managed to buy the wrong derailleur. I need one for Shimano 1×12 running 10-51 (or whatever the normal is). I think I need the M7100 (or for SLX level, 8100, 9100 if you’re feeling rich).

I managed to buy a M7120 instead which says 2×12 and max. 45 teeth on the low gear. I’m guessing I need to send it back – or will it work fine? Are they actually that different?

As you’ve said, they’re for the largely ignored 2x set-up that Shimano seem determined to keep alive. I guess it was probally a bit cheaper than average.

I’d guess the cage is too short (or long?) for a 51t cassette, you could try it, but if you do and it breaks, you can hardly send it back then.

I’d guess the cage is too short (or long?) for a 51t cassette

It’s not the cage that would be the issue. Both M7100 and M7120 have the same capacity of 41T. The M7120 mech will swing through a more shallow arc than an M7100, an arc designed to clear a 45T sprocket rather than a 51T.

M7120-M7100=M20

HTH.

Done the same thing but at XT level, the 2x (45t) mech needs to be bodged and fettled to work on 1x (51t) system. And you need to be lucky with hangers as well.

Thanks all (most 😉 )- pretty much what I expected.

I’ll get it wrapped up to go back. Might as well get the right bit. Every day is a school day.