Awl_TEQ
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They do make square sockets. I bought a set on ebay for exactly that same reason. They aren't called square sockets, there's a name for them that escapes me at the moment. They are 8 point sockets. That might help.
BTW, you won't be able to fit a mechanical temp sender there very well. It's too shallow for the probe in that area. The intake runner gets in the way. You'll need to stack up adapters and then not much of the probe will be in an area with coolant flow.
Eight point socket? Amaurer showed me a sears socket that's supposed to work. i already ordered a 1.5" adaptor from Summit. I'll post up a picture when I install it.
A standard twelve point socket would work fine as long as it fits tight - any multiple of four will work. Works on four sided set screws also.

