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post GM terminals is 5/16", not 8mm. It may be that a 8mm just happens to also fit, but these GM battery side-post bolts have been 5/16" since God was a small boy.
5/16" = 0.3125"
8 mm = 0.3150"
Only 2.5 thousands difference. Either will work ... I use a 6-point socket to ensure good engagement.
Without testing with a meter,, you most likely have a defective starter solenoid. If you just replace the solenoid ($50-$60),, you will most likely resolve your issue.
Had the exact same issue down to having to HOLD the ket to the start position.
If you bang the solenoid and it starts, you have the issue licked!!
Heres the starter schematic. As you can see,,,,, there are a LOT of components in the circuit that can cause the issue. The break in point is the Theft Deterrent Relay