M5100 rear mech - cassetter compatibility

There are different cages - one will only do 40t. Did you definitely buy the right one?

40t version

yeah, I bought that one. So not right for me.

Just assumed it would be the same as any other 11 speed 1x mech

The second link isn't a 40t max sprocket. It's a 40t max difference, 51 to 11t cassette.

Should have bought this one:

Shimano Deore RD-M5120 10 / 11 Speed Rear Derailleur SGS

which handles 11-42 or 11-46 cassettes and is 10 or 11 speed compatible.

I've got a few of those in stock if you are near Sheffield

The second link isn’t a 40t max sprocket. It’s a 40t max difference, 51 to 11t cassette.

Oh yeah, my mistake sorry

Although I do know that when I was buying an XT mech recently, there were two versions

I run a 42 max/min derailleur with a 11-36 cassette and it's fine - not perfect. Set the B-screw so that the derailleur is closer to the cassette than the line indicates as that line is for when you have 51t sprocket.

I'm running an M5100 with a 46t large sprocket cassette.
The cage pivot point is a long way from the centre of the top jockey wheel, so as the cage swings forward / back (due to changing chain length) it also moves the jockey wheel to follow the ramp of the cassette sprockets (rather than older mechs that pivot the cage at or near the jockey wheel and use the geometry of the parallelogram to follow the cassette).

The fudge to make it work is to play with chain length to pull the cage and hence top jockey wheel to the required position. It is really sensitive to chain length - too short and the jockey wheel will be miles from the cassette and vice versa.

What youve done is attached the cable to the wrong side of the clamp
ask me how I know!

What youve done is attached the cable to the wrong side of the clamp

Not sure about that, it's as per the dealer manual. Over the top, into the notch and then round and back up 

It is really sensitive to chain length 

I'd seen that with 12 speed mechs.

The chain is the right length per manual, did you make yours a bit longer?

I've got it changing up and down ok, but it's dropping when you back pedal

I’ve got a few of those in stock if you are near Sheffiel

Thanks for the offer, it's not urgent and I'm debating about whether to just get one of the 12 speed slx groupsets from cotic instead. I've already got a microspline free hub in the spares box.

You usually need to run the chain a bit longer to simulate the length it would be if you had a 51t sprocket. The 46t is approx the second sprocket on the big cassette so that is what you are replicating (just without a 51t to shift onto).

I was just in the garage and jockey to pivot is 25mm on an older Shimano 11 speed but 45mm on M5100.

as per the dealer manual. Over the top, into the notch and then round and back up

You are right