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I've ran several different 440 wedges, 426 Hemi and 400 with Elephant ears on the front with no midplate and a stock type trans. mount,they never cracked anywhere I've never had, seen or heard of a stock block crack like yours that wasn't frozen first

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I thought the same thing,that vertical crack in particular. I have only seen vertical cracks from frost.
I suppose the filler has absorbed water and/or is porus enough the expand on its own when frozen