7/8" and get the offset socket. Kind of like a crows foot. Regular O2 socket with the slot in it will be too close to the lines running along the firewall and you will not be able to get a universal, or extension on it. Places like Autozone will loan it, with a small deposit. Be prepared to be scratched up like you been in a cat fight. Need small hands with long fingers to reach down to unscrew it when loose and to hand tighten.
