Rear derailleur cage hits largest cog on cassette

Question

I got a new rear derailleur for my wife's bike after a stick turned the original one into a twisted mess of metal. The derailleur is a Shimano Acera, the cage is slightly longer than the original.

The rear cassette is a Shimano MegaRange with 7 cogs, the largest being 34T and quite a bit larger than 2nd.

When engaged on the second cog, the inner cage plate begins to rub on the largest cog. Getting into the largest cog isn't possible due to interference with the cage. I tightened down the B screw all the way, but there just isn't enough room.

The inner part of the cage on the original derailleur is a bit shorter than the upper jockey wheel. On the new one, the inner cage goes a bit beyond the jockey wheel.

Is there some adjust I'm missing or a different part I should have gotten? Worst case, could I cut the inner cage plate back a bit for clearance?

EDIT:

I found the tiny stamping on the old deraileur, its a RD-M340. The new on is a RD-M360. Both claim to handle a 34T rear.

With a little manual intervention, I can get the chain up on the top cog and it turns smoothly.

The old (dirty) deraileur:

The new one in low:

The new one hitting the gear:

Answer 1

The inner plate seems to be on upside down. Not sure why but if it's second hand, maybe someone dismantled it to clean it the put it back the wrong way round.

If you need to swap it around you just need to undo the screws holding the jockey wheels in and flip it, then reinstall them. Its a bit tricky with the chain tension, but if you can do it without removing the chain, all the better.

Maybe drop the chain off of the chainring at least, to loosen some of the chain tension.

Make sure you get the chain routed correctly around the little tab on the inner plate, which is about halfway between the jockey wheels.

Also check the jockey wheels go in the right place and spin the right way; thanks to Carel in the comments, “If there's a difference between the top one and the bottom one there should be an inscription and an arrow with the sense of rotation.”

Answer 2

It sounds like your derailleur does not have capacity for a 34 tooth cog. Are you sure the B tension screw is right ? They can function opposite to expectation, depending on the model, you may need to loosen it not tighten it.

A quick search suggests all Acera rear mechs should handle 34 Tooth.

"Shimano Acera M360 7-8 Speed Rear Mech" which seems to be a common result in a search, then it is rated for "28-34 tooth largest sprockets, with a capacity of 43 teeth"

"Shimano Acera M390" seems to have no published specs, but is unlikely to have a smaller maximum than the other models.

"Shimano Acera RD-M3000" supports 36T maximum, with a capacity of 45.

Cutting up a rear mech is probably not going to help you, and would be a very last resort.