Well, one of my Optima batteries went kaput so I decided to take a stab at fitting the infamous Sears Diehard Platinum 31M. Some people have been successful getting this battery to fit, others have had difficulty. I thought I'd post my experience/thoughts on the getting it to fit properly. The Platimum 31M has plastic fins that surround the left and right sides of the battery. My guess is that these provide a layer of protection around the battery more than do serve as heatsinks. Anyway, I took the drivers side battery tray out of my vehicle and started playing around with the fitment. I noticed that the fins around one small corner of the battery (bottom, rear, and closest to the fender) were hitting the tray and preventing the battery from sliding in. I used some wire cutters and trimmed the first three fins on the bottom corner of the battery (see pic). Notice that you only need to trim between 0.5" and 2" of each of these three fins. After that, the battery slid right into the tray like a glove. I reinstalled the battery tray, slid the battery in, made a new battery hold down bar with some aluminum stock from Orchard Supply and Hardware, and voila!!! It's done! Hope that's helpful for anyone thinking about installing one of these.