I disagree RonInSD.
According to section 19.5 "The use of a rotary molded fuel cells not having a bladder, or not contained in a metal can, is allowable in those cars that do not require the use of a fuel cell, but where they are an allowed option."
The cells that Ron Learp found don't meet this description, if I remember correctly how they're built. I think they are a metal can with foam inside and no bladder (rotomolded or otherwise).
I'm not sure what I would do if you brought a car with one of these to me for an annual inspection. At least the rotomolded cells could stand some crushing. I think the molded cells in a metal can (which don't generally have foam inside) are a better choice if you can't swing an FIA cell.
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Ty Till
#16 ITS
Rocky Mountain Division