Cheap Derailleur Spring Tension Mod

In this older post, I showed you how to change the cage tension on Shimano derailleurs with aluminum knuckles. Those are easy. The ones with plastic knuckles are more difficult since there's no direct way to do this. This is still possible, however. Here I have an Acera derailleur that could use a little more tension:

On the side there is a small plastic cover that needs to come off. I like to drill 2 small holes one either side first:

Then I use 2 small screwdrivers on either side to pry it out (be careful, don't let the screwdrivers slip). This is a little easier if you have the derailleur locked in a vise, but anyway here is the result:

With that plastic cover out, you'll see a small metal retaining clip holding the knuckle together. Rotate the clip until the ends are facing you like this:

Now push the clip inwards with 2 small screwdrivers. When you push it far enough, the cage will simply pop out:

Now comes the fun part. You need to drill another hole for the end of the spring to fit into:

Centerpunch the location for the new spring position, then start drilling:

The drill bit size I used was a #52 (.063"). You can also use a slightly larger size, since the spring was a tight fit. It's important to keep the drill straight. I also used some oil on the drill bit to keep it cool. I didn't apply any extra pressure on the drill since the bit is so small and fragile, I just let the weight of the drill do all the work. Here's the final result:

Now clean away the metal shavings and grease the center post. Fit the spring into the new hole you just drilled:

Now slide the derailleur back together, making sure the other end of the spring is secure inside the hole on the other side. You have to twist the cage back a little until the tab is resting behind the stop on the knuckle, then push the assembly together. Finally, reinsert the E-clip:

The E-clip goes back in more easily than it comes out, just shove it back into its grove with a small screwdriver. The last step is to reinsert that plastic plug:

Now enjoy that extra chain tension.