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Tearing it down starts with driving out the countershaft. The extension housing needs to be unbolted and rotated to the position shown to access it. In aluminum-case overdrives like this one, the countershaft is designed to float in the case and eventually pounds out the case bore, effectively trashing the tranny. Note that the shaft is keyed to the case on the extension housing side, meaning it must be driven rearward to get it out.