12-speed Rear Derailleur installation and adjustment guide

Updated May 11, 2024 by BetterShifting Terry

With every new generation of Di2, Shimano tweaks the derailleurs and their installation process. Twelve-speed Di2 road rear derailleurs are very similar to their 11-speed siblings, with a couple of changes to make your life easier.

Tools and prerequisites

Before you start, make sure you have these tools nearby:

And yes, you can install the rear derailleur without a torque wrench or the B-gap tool, but they definitely help.

Want to know what tools I use? I have listed them on my Amazon page.

You’ll also need the derailleur itself, and the small parts that come with it:


Finally, you’ll need to have the BT-DN300 battery installed, as well as a 12-speed chain and cassette, to complete setting up the derailleur.

Optional: install the wire holder

Twelve-speed Di2 front derailleurs come with an optional wire holder. It’s a little piece that simply clips onto the rear derailleur. If you’d like to use the wire holder, I recommend attaching that to the rear derailleur now. It’ll be easier than doing so with the derailleur on the frame.

To attach the wire holder, simply push it into the bracket axle as shown in the images below.

install 12sp rd wire holder

Attach the derailleur to the frame

Before you attach the derailleur to the frame, take a good look at the derailleur hanger and determine whether it is a standard hanger or the direct mount type.

Direct mount hangers appear in line with the seat stay, while a standard mount will make a sharper bend down.

Derailleur mount types

Standard mount

On a standard mount frame you install the derailleur with the B2-link plate attached. It should come this way out of the box. If the B2-link plate is not installed, do so now – it is colored green in the image above.

Apply a small amount of grease to the mounting bolt and secure the rear derailleur to the frame. Before tightening it fully, make sure the stopper plate touches the B-tension stop, with no gap between them.

Install rear derailleur to the hanger

You will need a 5 mm hex wrench to do this and the recommended torque is 8 – 10 Nm.

Direct mount

Is your derailleur hanger of the direct mount type? Then remove the B2-link from the rear derailleur so that you can mount it directly onto the frame/hanger.

You do this by removing the bracket axle using a size 5 hex wrench. Remove the link and attach the derailleur to the frame using the bracket axle.

Rear derailleur direct mounted

Connect the EW-SD300 electric wire

If you haven’t done so already, route the EW-SD300 Electric Wire from the battery to the rear derailleur. You can use any of the battery's ports. The only condition is that there must be something in the middle port - either a wire or a dummy plugs will do.

Want to use the derailleur’s wire holder? Then route the wire through that too. Simply push it behind the clip and it’ll stay put.

Next, take the rubber cover that came with the derailleur and slide it over the other end of the wire.

Slide it just past the plug, and then use the TL-EW300 wire tool to push the wire into the port on the derailleur. You should feel it click or snap into place.

With the wire secure, slide the rubber cover down, making sure it covers the wire plug and the port on the derailleur.

Install the derailleur port cover and wire

That’s it, the derailleur is now connected to the bike. Now put the rear wheel back on the bike and install the chain.

If you haven’t sized your chain to the correct length yet, read this page on chain sizing.

Finish setting up the derailleur

With the derailleur installed to the frame and connected to the rest of the system, there are only four steps to set up a 12-speed rear derailleur is done through four steps. They are:

Adjust the B-gap

The B-screw is used to set the gap between the derailleur guide pulley and the largest cassette sprocket. You’ll want this gap to be relatively small, without having the guide pulley touch the sprocket cogs.

To get started, first shift the front derailleur to the smallest chainring. Shift the rear derailleur to the largest sprocket, or easiest gear.

Shimano’s dealer manual states that you should be turning the crank arm in reverse as you do this. The training videos don’t mention this at all, so it probably doesn’t matter all that much.

Turn the B-screw to move the guide pulley

Next, we’ll turn the B-screw to adjust the gap between the guide pulley and the largest sprocket. The recommended gap depends on the cassette used.

This is Shimano’s guideline for Dura-Ace and Ultegra:

  • 11-28T - 18mm
  • 11-30T - 14mm
  • 11-32T - 10mm
  • 11-34T - 6mm

And these are the recommended values for 105 Di2:

  • 11-34 - 10mm
  • 11-36 - 6mm

Alternatively, you can use the GG adjustment gauge. This should be included with the rear derailleur. If it isn’t, and you’d like to buy one, the part number is Y3GK89600.

Micro-adjust your 12-speed rear derailleur

If you have access to a 3D-printer, you can try printing one. Chris Heerschap published a model of the tool on ThingiVerse.

You set the adjustment gauge on the plate as shown below, and then turn the B-screw to align the tip of the sprocket teeth with the correct line.

Set the B-gap tool

If you’re using a 30T cassette, use the top line. If you’re using a 34T cassette, use the lower line. For 28T cassettes you’d use the top of the adjustment gauge itself.

Note that the 105 rear derailleur has different optimal distance settings and therefore also its own specific adjustment gauge tool. You can not use the Ultegra / Dura-Ace tool with 105 Di2.

Micro-adjust the gear shifting

Next, you’ll have to fine-tune the gearing, using micro-adjustment. I'll show you how to do this on the bike, but you can also set 12-speed rear derailleur limits using the mobile app - E-Tube Cyclist.

First, shift the rear derailleur to the 5th gear from the largest sprocket, and shift the front derailleur to the small chainring. This will ensure the chain line is as straight as possible.

Adjust your RD starting in the 5th gear

Press and hold the rear derailleur button for about 2 - 5 seconds to enter adjustment mode. When in this mode, the LED lights up yellow.

Once the system is in adjustment mode, press the button highlighted below, while turning the cranks. This will move the rear derailleur toward the larger sprockets, in tiny steps.

Adjust the RD towards gear 4, until it rubs

Keep doing this until the chain touches the 4th sprocket, and a slight rubbing noise can be heard.

At this point, you’ll have to back off the rear derailleur a bit to move it to its final position. Press the button shown below five times.

Move the RD back 5 steps, towards gear 5

Press and hold the rear derailleur button for about 0.5 seconds, and you’ll exit adjustment mode.

The derailleur should now be set correctly. This is just a starting point though, so shift through all the gears and double-check that they all work well. Make further adjustments if necessary.

Note that at this point you haven’t set the derailleur limits yet, so be careful not to shift the chain off either end of the cassette. Also, if the derailleur doesn’t shift onto the last gears, that is OK at this point. As long as all the other gears are shifting fine, don’t worry.

If you find that your derailleur shifts well in the middle of the cassette, but performance degrades at either end, consider checking your derailleur hanger alignment.

Dave Rome wrote a really good guide on derailleur hanger tools right here: derailleur hanger alignment tools on CyclingTips.com.

Set the low limit

With the derailleur shifting set up, let’s now set the low limit. Shift the rear derailleur to the largest sprocket (easiest gear). Now use a size 2 hex wrench to tighten the low side stopper bolt until it just touches the low side stopper. This is the bolt with an L next to it.

It’ll look similar to this:

The derailleur's low limit stopper bolt

If you’re confused and can’t get it right, just send me a message and I’ll do my best to help.

To check whether your limit setting is correct, shift the rear derailleur down three gears and then back to the largest sprocket again. It should shift to the easiest gear just fine.

If it doesn’t, relax the limit screw a bit, and try again.

Set the high limit

Now we’ll set the high stopper bolt.

Shift the rear derailleur to the smallest sprocket or hardest gear. Tighten the top stopper bolt with a size 2 hex wrench until it just touches the top-side stopper. This is the bolt with an H next to it.

Can't shift to the smallest sprockets?

If you're using 12-speed Dura-Ace derailleurs, you probably cannot shift into the smallest sprockets if the front derailleur is in the small ring. The solution is simple: shift the front derailleur to the big ring and you're good to go.

Turn the top-side stopper bolt counterclockwise one turn to loosen it and allow for the rear derailleur to overshift.

Set the stopper bolts

Note that if you overtighten either stopper bolt the derailleur will not stay in that highest or lowest gear and shift back within five seconds. When this is the case you'll also hear some clicking or ticking, indicating that the derailleur is blocked from fully completing the shift.

If this happens to you, loosen the top side stopper bolt a bit, and check the shifting again.

Shimano says that the following could happen, if your limits are set too tight:

  • Chain noise does not stop.
  • The battery level drops quickly because extra load is being placed on the motor.
  • The motor may be damaged due to overload (irreparable).

All done!

Your rear derailleur should now be installed and set up pretty well. If you're having problems, or just cannot get it right - leave a comment or send me a message and I'll do my best to help you!

BetterShifting Terry

About the Author - BetterShifting Terry

I enjoy playing with bike tech - both bike building and wheel building, bike maintenance and of course, Di2. Besides writing content and working on the technical side of BetterShifting, I also work as a Software Developer in The Netherlands. Read more on the About this site page.

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