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I was in the process of upgrading my OEM battery (group 35) on my STI, but I had an issue with the battery hold-down not properly fitting the width of many AGM group 35 batteries (Duralast, Autocraft, Optima, Champion, Bosch). These AGM batteries, although specified as group 35, are not as wide as the flood-type of batteries by 7/16 inches. I looked for a bracket that would fit these batteries, but unfortunately, most of the Subaru WRX retailers only sold hold-downs for the same width as flooded-type batteries. Secondly, some auto parts stores sell some "universal" hold-downs, but these too don't fit that well on top of the battery.

After thinking of ways to use the OEM bracket and a spacer, I finally found a machine key that fit the gap perfectly between the battery and the hold-down. A machine key is basically a rectangular piece of metal used in assembling transmissions. You can find these for sale at a hardware store, in the section that carries all the various bolts, nuts, washers, and other specialty hardware. Here is a picture of one that I used:

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Here is a picture of the battery hold-down on the OEM battery:

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As you can see, the battery hold-down on a group 35 battery (flood-type) fits snug on top of the battery. One would think that all group 35 batteries would share the same width, but AGM batteries are not as wide.


Adding the 7/16" x 2" machine key between the clamp and the top edge of the battery provides a really snug and secure fit for an AGM group 35 battery. Once tightened, the battery is firmly secured to the frame using the OEM tie-down J-hooks. I have driven with this setup for at least one month through many types of roads and everything is still rock solid. The Optima batteries do not have a little lip on the top edge to rest a machine key, so you may need to look for some alternate way to mitigate the gap between the hold-down and the battery.

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The battery I chose is a Bosch Platinum Series AGM Group 35. Due to work and leaving the car in the airport garage for weeks, I needed a battery that had a larger reserve capacity than the OEM battery, and a battery that was more maintenance-free and reliable in adverse weather conditions.

I hope this helps anyone looking for a way to securely hold the newer AGM batteries with the current OEM battery hold-down.
 
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