Aluminum or plastic battery tray? This might not apply to your situation, but I just upgraded my 5-yr old AGM starting battery from a grp 27 to a grp 31. My boat has a recessed plastic tray (2002 vintage) designed for a grp 27. What I found out was that the top hold-down part of my tray would not fit that big bad Diehard 31M (same as an Oddysey 31M) but it would fit on an Optima grp31. I had to do a very small amount of carving to a small section of the inside edge of the bottom part of the tray to get the Optima to fit, but it saved me from having to tear the old tray out completely. The Optima probably wont last as long as the Odyssey or Diehard, but it is rated way better than my grp 27 AGM.
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