What size socket to remove Oil Sending Unit?

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I'm trying to replace the oil sending unit on my 98 Camry, which is on the front of the engine above the crank shaft position sensor. My 22mm deep socket fits the new one perfectly, but I can't get the old one out! I removed the alternator to have better access to it and was able to get the wiring harness off, but my 22, 23 and 15/16 deep well sockets won't go on the old one. Two things about the old one: 1. Looks like it's either been stripped or that its actually round with 2 flat sides to grab (not like a std. 6-sided nut). 2. Its seems like its a good bit bigger (diameter) than the new one. Does anyone know what size it is, or if it needs a special tool to remove it? I asked AutoZone if they had a tool for it, but they said there wasn't one. I've tried getting a set of channel lock plier on they to see if I could turn it, but had no luck. Any help would be great. Thanks